You quoted this sentence:
If there are 2 modals in the clause the finite one may come before or after the 2 infinitives.
This rule applies only to clauses that have 2 modal verbs. But here we have two clauses, each with its own modal verb.
- main clause:
Man muss ein Jahr lang getrennt sein.
You have to be separated for a year.
- subordinate clause:
etwas muss geschehen, bevor man sich scheiden lassen kann.
something must happen, befor you can get divorced.
A subordinate clause can not be a full sentence on its own, it alway must be attached (subordinated) to a main clause. And there is a second difference between main clauses and subordinate clauses: They have different word order.
The finite verb in a main clause of a German sentence must always be located at the second position of the clause. Exceptions from this rule are extremely rare. (In jokes the verb is often located at position 1, but this here is not a joke.)
But in subordinate clauses the finite verb must always stand at the very last position. (Again there are some rare exceptions.)
If you have more than just one verb in a clause, only one of them is the finite verb (the one that must be declined to match to the subject in number and person). All other verbs will be used in their infinite form (i.e. not declined).
There are three classes of verbs that you have to think of:
German has 6 modal verbs
dürfen, können, mögen, müssen, sollen, wollen
German has 3 auxiliary verbs
haben, sein, werden
All auxiliary verbs can also be used as full verbs.
All other verbs are full verbs
gehen, trinken, schlafen, ...
When they appear at the end of a subordinate clause, they appear in this order:
full verb - auxiliary verb - modal verb
Es kann durchaus sein, dass er um acht gegessen haben will.
And because the modal verb it the last one, it is the finite verb.
You can convert any subordinate clause into a full sentence just by moving the finite verb from the very end to position 2: