Ideally is for example 2 2S 25C 5000
's, this being all the same aswell as the packs age and how many runs they've had.
If you have the evx2 in their 2X2S is the maximum. 4S total.
with brushless you can for example have one 2S 25C 5000
and one 3S 25C 5000
if they are used as 5S from the start.
You can not safely go out with for example one 2S 30C 3700
and one 2S 25C 8000
or one or the other will die.
Hope this is the info you needed.