Are two smaller septic tanks equivalent to one large tank?
Check out this answer here: diy.stackexchange.com/a/1712/928 

It sounds like you have a 800 gallon septic tank, and an 800 gallon black water tank. Which means you should pump for 800, not 1600.

Cautionary note:

In some rare circumstances, most notably the rocky lake district of Muskoka in Ontario, weeping beds are illegal (lake water contamination), and the second tank is actually a holding tank.  In this situation, you need to pump out the tank fairly often, or it will fill up and overflow.

If you have a nice lawn that stays green without watering in the dog days of summer, then you most likely have a weeping tile bed.