The process outlined by Straight6TT is pretty common. Just make sure you keep the carbonation level at or under 1.8 vol or you'll have trouble getting a decent pour. Most people are happy with 1.2-1.4 vol. You'll probably want to serve around 25-35 psi, and might want to do a little experimenting to figure out what pressure gives you the best pour.
I have a similar setup, 2 perlick C02 and 2 stout taps on beer gas. I've tried a few ways to carb up what I push with beer gas. Here what I know do. On keg day I purge/seal with beer gas. When I put it into the fridge, I carb it up with beer gas, and I push with beer gas, all at about 20psi.
Now, it probably costs me a bit more but I have run into a few times where I purge with c02 and carb with c02 and then push with beer gas and it's all just foam. I have also had it come out fine.
But for me, I like my beers a little flat anyways so by just using beer gas the whole way I get that nice cascade, no chance of over carb and it's good to go to smooth town.
As long as you're carbing to the same level, it shouldn't make any difference. It takes much longer to carb with beergas, and uses a more expensive product, which is why most people use CO2 instead. What was your method for carbing with CO2?
Are you guys using 70%/30% or 75%/25% beer gas, and could you even really tell a difference?
I don't have a nitro set-up myself, but have helped set up and troubleshoot several of them. It doesn't make much difference, but I'd suggest the higher CO2 content if you have a choice, and here's why.
When using 100% CO2 it's important to keep the serving pressure set to whatever equates to the carbonation level of the beer, or the carb level will change over time to match the serving pressure. With nitro the carb level doesn't change much even if your serving pressure doesn't match the carbonation equilibrium pressure, so it's not nearly as important. Instead of using the equilibrium pressure to serve, most people play with the pressure until they get the pour they're looking for. In most cases this pressure ends up being quite a bit lower than the equilibrium pressure, and the keg actually loses a small amount of carbonation over time. It's usually not enough to be noticeable, so it's not a big problem, but this effect can be minimized by keeping the serving pressure closer to the equilibrium pressure, which using a higher CO2 content blend usually helps with. For example, using a 25% CO2 blend at 38° to maintain 1.4 vol requires ~43 psi. To keep the same equilibrium with a 30% CO2 blend only requires ~33.5 psi.
If you're interested in calculating what the equilibrium pressure would be for a beergas blend, this calculator is what you need. Like I said though, that likely isn't the pressure you'll end up using.
http://mcdantim.mobi/easypsig.html
