These two batteries will deplete in voltage equally, at least in theory, right?

So, these can be charged together, right?

 

What 2 batteries?

 

Suppose you have a 24V system. You are creating 24V by placing 2x 12V batteries in series. These batteries aid the equipment to start.

Over time their output voltage will decrease. Will it decrease symmetrically on both batteries? Can they be charger simultanously?

 

Both the cells will not necessarily have same internal resistance....
So, energy lost can be different..

 

If the batteries are identical in age, make, and capacity you'll be OK.