My dictionary says "lebenspendend" and "Leben spendend" but I cannot find lebensspendend double s compound. I suppose it's not a variant.
As @infinitezero already said DWDS lists lebensspendend as a variant of lebenspendend, so you may say it has the same meaning.
But it gets interesting, when you take a look at Google's ngram Viewer, which displays a graph showing how those words have occurred in a corpus of books.
As you can see since the 1980's the variant seems to be more common than the original.
Duden does not list the word, however DWDS does list it as a Nebenform (variant).
The question asked was actually:
Can lebenspendend also be spelled lebensspendend, i.e. with an additional s?
To answer this: there is no final rule-based answer. The writer is at liberty to write it either way. For my taste, lebenspendend would be slightly more poetic, lebensspendend sligtly more prosaic.
As for the productivity of suffigations with -spendend: you can glue it to other substantives with suitable meaming as it pleases you:
and so on. (I am sure most of them will not be registered in dictionariesm but this does not make them invalid or inexisting.)
Most of them would be spelled without an additional "s". An "s" could be added, though, to labungsspendend and heiterkeitsspendend. Which leads to the question: when can "s" be added, and when not? I suppose there is some complicated rule, but at the moment I am not able to present it.
You just used the word lebensspendend, so: here it is! You used it, so it exists.
The goal of each language is to transport ideas. If a speaker/writer uses a certain word, and if the listener/reader has a high change to guess which idea was in the speakers/writers mind when he/she used this word, then the word not just only exists, but it even does its job: It transports an idea.
There is nothing like an official bureau that can say that one word exists, and another doesn't. There are just editorial departments who collect and list words that are more or less often used. But sometimes they list old and outdated words, that no longer are in use (but still exist), while they fail to list brandnew (but also existing) words.
Since the question of officially existence is not answerable without a definition of official regarding the use of a language, I consider your question being meant in the way of:
Would the word lebensspendend be considered valid German?
DUDEN recomments the spelling Leben spendend and actually that's the meaning of lebenspendend - live-giving.
If something spendet Leben, it is lebenspendend. If people are donating blood, they are blutspendend, ...
I have no idea, why there should be an additional s at the end of Leben. I think, nobody would ever say blutsspendend. It just doesn't make sense whatsoever.
So I think, that's a (according to @mtwde's post common) mistake arising from the structural resemblance to the word lebenserhaltend.
But lebenserhaltend is a different story.
- Lebenserhaltende Maßnahmen (life support measures) are Maßnahmen zur Erhaltung des Lebens.
- Lebenserhaltende Funktionen (life support functions) are für die Erhaltung des Lebens notwendige Funktionen.
So there is a genitive in there justifying the additional s.
That doesn't apply for lebenspendend and therefore imho
lebensspendend is simply wrong.