Yes, of course, you can do this and say

Je stärker ich sei, desto erfolgreicher wäre ich.

The only problem is that sentences like this require very specific contexts to be used in, otherwise they sound weired - up to the level that people think they are wrong; which they technicall aren't, given the right context.

Your sample sentence uses two different forms of subjunctive, the "sei" for indirect speech, and the "wäre" for irrealis.

A more easily allocatable sentence would be:

Ich mache neuerdings Krafttraining, denn mein Trainer hat mir gesagt, je stärker ich sei, desto erfolgreicher werde ich sein. (Or ... desto mehr Erfolg werde ich haben.)

This is pure indirect speech. Usually people do not use this form of indirect speech in oral communication. They would rather say:

Ich mache neuerdings Krafttraining, denn mein Trainer hat mir gesagt, je stärker ich bin, desto erfolgreicher werde ich. (Or ... desto mehr Erfolg werde ich haben.)

This is indicative. Note that werde hier is not subjunctive but future! That's a bit confusing, as the indicative and subjunctive forms are identical.

In formal, written communication the use of indirect speech is standard, though.

Now for integrating also the irreal or conditional form, we have to go to great lenghts with our fantasy to come up with a scenario where such a sentence could make sense in practice.

Ich mache neuerdings Krafttraining, denn mein Trainer hat mir gesagt, je stärker ich sei [indirect speech], desto erfolgreicher wäre [conditional] ich im Falle einer Nominierung für die Auswahlmannschaft.

This uses an additional conditional, and thus the wäre makes sense here.

Yes, of course, you can do this and say

Je stärker ich sei, desto erfolgreicher wäre ich.

The only problem is that sentences like this require very specific contexts to be used in, otherwise they sound weired - up to the level that people think they are wrong; which they technically aren't, given the right context.

Your sample sentence uses two different forms of subjunctive, the "sei" for indirect speech, and the "wäre" for irrealis.

A more easily allocatable sentence would be:

Ich mache neuerdings Krafttraining, denn mein Trainer hat mir gesagt, je stärker ich sei, desto erfolgreicher werde ich sein. (Or ... desto mehr Erfolg werde ich haben.)

This is pure indirect speech. Usually people do not use this form of indirect speech in oral communication. They would rather say:

Ich mache neuerdings Krafttraining, denn mein Trainer hat mir gesagt, je stärker ich bin, desto erfolgreicher werde ich. (Or ... desto mehr Erfolg werde ich haben.)

This is indicative. Note that werde hier is not subjunctive but future! That's a bit confusing, as the indicative and subjunctive forms are identical.

In formal, written communication the use of indirect speech is standard, though.

Now for integrating also the irreal or conditional form, we have to go to great lenghts with our fantasy to come up with a scenario where such a sentence could make sense in practice.

Ich mache neuerdings Krafttraining, denn mein Trainer hat mir gesagt, je stärker ich sei [indirect speech], desto erfolgreicher wäre [conditional] ich im Falle einer Nominierung für die Auswahlmannschaft.

This uses an additional conditional, and thus the wäre makes sense here.

Yes, of course, you can do this and say

Je stärker ich sei, desto erfolgreicher wäre ich.

The only problem is that sentences like this require very specific contexts to be used in, otherwise they sound weired - up to the level that people think they are wrong; which they technicall aren't, given the right context.

Your sample sentence uses two different forms of subjunctive, the "sei" for indirect speech, and the "wäre" for irrealis.

A more easily allocatable sentence would be:

Ich mache neuerdings Krafttraining, denn mein Trainer hat mir gesagt, je stärker ich sei, desto erfolgreicher werde ich sein. (Or ... desto mehr Erfolg werde ich haben.)

This is pure indirect speech. Usually people do not use subjunctive in oral communication, but in formal written communication it is standard.

Now for integrating also the irreal or conditional form, we have to go to great lenghts with our fantasy to come up with a scenario where such a sentence could make sense in practice.

Ich mache neuerdings Krafttraining, denn mein Trainer hat mir gesagt, je stärker ich sei [indirect speech], desto erfolgreicher wäre [conditional] ich im Falle einer Nominierung für die Auswahlmannschaft.

This uses an additional conditional, and thus the wäre makes sense here.

Yes, of course, you can do this and say

Je stärker ich sei, desto erfolgreicher wäre ich.

The only problem is that sentences like this require very specific contexts to be used in, otherwise they sound weired - up to the level that people think they are wrong; which they technicall aren't, given the right context.

Your sample sentence uses two different forms of subjunctive, the "sei" for indirect speech, and the "wäre" for irrealis.

A more easily allocatable sentence would be:

Ich mache neuerdings Krafttraining, denn mein Trainer hat mir gesagt, je stärker ich sei, desto erfolgreicher werde ich sein. (Or ... desto mehr Erfolg werde ich haben.)

This is pure indirect speech. Usually people do not use this form of indirect speech in oral communication. They would rather say:

Ich mache neuerdings Krafttraining, denn mein Trainer hat mir gesagt, je stärker ich bin, desto erfolgreicher werde ich. (Or ... desto mehr Erfolg werde ich haben.)

This is indicative. Note that werde hier is not subjunctive but future! That's a bit confusing, as the indicative and subjunctive forms are identical.

In formal, written communication the use of indirect speech is standard, though.

Now for integrating also the irreal or conditional form, we have to go to great lenghts with our fantasy to come up with a scenario where such a sentence could make sense in practice.

Ich mache neuerdings Krafttraining, denn mein Trainer hat mir gesagt, je stärker ich sei [indirect speech], desto erfolgreicher wäre [conditional] ich im Falle einer Nominierung für die Auswahlmannschaft.

This uses an additional conditional, and thus the wäre makes sense here.