We have prenuptially agreed to keep savings separate. We also agreed to split all taxable income 50/50 to make it easy to file joint for taxes without setting up some elaborate accounting scheme to figure out who's responsible for how much of the taxes given their income. We have a joint account that receives all this income. This account pays shared facilities. We compromise on what falls under this. For instance, I pay half the car even though my use of the car is <5%. DW pays half the internet even though I use 99% of it. DW pays her own cell phone. I pay for tools even though they are used to fix the house. Our RV is bigger than what I wanted and smaller than what DW wanted.
Surplus in the joint account gets split 50/50 and goes into our respective savings accounts. This made sense at the time (or the marriage) and was done to make it easy (no hard feelings "after everything I've done for you") to split up in case we fall into the 40-50% of all couples who eventually divorce. We wanted to make it fair so that neither side would feel taken advantage off---so that's the perspective I have for the reasons for the setup.
Now, in terms of splitting income, I'm still in the lead. Historically I've contributed more cumulatively speaking than DW has although given that I now make less than DW, this will eventually change. At this point, we may renegotiate. In particular, I would feel like I'm keeping DW back from saving due to the 50/50 split even as I already got enough money.
I totally agree with S in that there's a gender bias. While I can pay all my expenses and almost all of DW's expenses with a SWR out of my savings, I sometimes get comments from internet clowns that "my retirement = DW working". Incidentally, these guys would have no issue if I was the one working and DW stayed at home. Gender bias at its finest. I'd dearly love for DW to become FI too (will happen in a few years) and stop working just to shut this group up.