How do snakes deal with return outputs going to monitors?
A snake is just a bundle of mic/line cables. Whatever you put in one end comes out the other end (within reasonable limits).
do both splits accommodate the max number of outputs/returns?
Both splits from WHAT? Do you think that a snake performs splits? (trying to understand what you mean by "splits" here?)
Are they split 50/50 meaning I need to potentially accommodate for the max amount of outputs I'd need for a single return-split (i.e. more than I need, depending on ratio of Stage Monitors to PA/Audience Monitors)?
This isn't working very well as a generic question. If you are talking about line-level outputs from modern equipment, they can typically drive several destination inputs because of the typical low output impedance and the high input impedance.
If Splitters with 40m+ distance isn't possible
40m is not a particularly long distance for modern audio equipment. We sometimes run 100s (or even 1000s) of feet of mic and line-level runs for large events. The ability to drive distances has everything to do with the output impedance of the source. Since "splitters" are undefined here, no definitive answer to your question is possible.
should I be aiming for a second stage box to extend this? Or is there an easier way to extend the length?
We don't have any information necessary to establish whether the source can (or can't) drive the distance. A second stage box will do nothing to extend anything unless you mean some sort of powered gadget with amplifiers inside.
Can I get Multicores with split outputs? Is what I just said effectively the same as a splitter?
Snakes and splitters are commonly separate things. It is not clear why you would want to combine them. It isn't really clear what you are talking about splitting here? Mic levels or line levels? Transformers are typically used for mic-level splitting, but you can typically simply parallel loads on a line-level output. Of course there are many variations here and use of transformers for isolation of line level signals may be necessary even if they are not necessary simply for the "splitting" function.
Rather than using a splitter, is there a way to rebound the signals that I want in the Stage Console from the FOH console?
Mic level signals? Line level signals? Your question is not detailed enough to answer.