I know this has been a much debated topic over the years, and has been somewhat rejected (understandably so) due to it just being wayyy too hard to implement! I can't imagine the stress on the coders to have to figure out how to have two halves of a lion, and make it work functionally. As much as I'd love to have 2-in-1 lions that can double their potential breeding outcomes, I just can't see it happening. After some brainstorming, I've come up with a way to make it work that won't be hard at all to implement.

My Solution:
If Chimerism is rolled, there will be a generated overlay that is picked randomly from a pool (all would have more or less the same chance of rolling). The overlays work identically to all of the other posed mutations. (e.g. Piebald/Patches/PF/Albinism etc..) The pool would consist of several bases (maybe chosen via a poll to see what the community wants!?), and for each base, there would be 2-3 marking variants. These wouldn't be actual "bases" or "markings", just images on top of the existing lion.

No, the "base" on the overlay WOULDN'T be able to pass onto the cubs. (Unless devs decide to make that happen, but that would probably be way too difficult.)

Examples
Overlay (sorry it's messy, it was semi rushed)

Overlay On Lion (I made a mistake in doing this one, I accidentally duplicated the lineart layer and didn't notice in time, which is why it's so dark)
yeah that's my lion lol (it would be underneath the lineart file, on top of the base and marking files, just like all other posed mutations)

ANYWAY, I hope you like this suggestion and take it into account :)