Inrush current seems to be a big bugaboo

Seem to have time to kill. Lets try for some numbers.

Here's the fuse data sheet
https://www.littelfuse.com/~/media/a..._datasheet.pdf

I use a 7.5 amp fuse on each battery.

Assuming the battery full and empty voltages differ by 1 volt.
The current with both batteries on the bus also depends on the wiring resistance. If there is 0.1 ohms, then the fuse current is 10 amps.
at 25C, that could blow it in a few seconds.

The I2T rating for the fuse is 60. That is measured over 8 milliseconds. So I think that says the fuse will blow quickly with 86 amps.

Seems like unless you have really good wiring or a big charge difference, quickly switching make before break should be ok.

That matches your experience with the Stemme and also the Walmart run at the recent contest where the switch was left with both batteries on the bus for a while.