Related: How often do septic tanks typically need to be pumped?
Our home has two 800 gallon tanks next to each other, from the previous plumbers drawing it looks like they are connected. 
I was looking at http://www.nesc.wvu.edu/pdf/WW/publications/pipline/PL_FA95.pdf
And they say 1500 gallons for 2 people should be pumped every 9.1 years. Should I use that guide for my combined tank size or should I follow whatever is closer to 800 gallons?
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.
