Putting some load on battery A and then battery B vs
putting the same load on battery A+B.

If there is charging involved during the load, then
the second would mean that neither battery would be
drained to the level of each of them in the first.

Hence, the second is "better" since it is always
"better" to drain a battery less rather than more.

If no charging is involved during the load, then
it doesn't matter.

Richard