We have a similar split in managing our finances, we too are both self-employed, and I happen to be about your age too (my wife is a few years younger), so this was even more interesting to read than it would have been anyhow.

Your point about the $20k being spent before it came in is what resource managers call "encumbered." And if you know how much of next month's (or next year's) income is already encumbered, or more colloquially "spoken for," then you know how much "play money" you have left when more money comes in than you expected.

In our case, I worked savings and investments into oour budget, so there's almost never any unexpected excess. However, that doesn't mean there's no money for play, since there are generous amounts in that part of the budget too...