From what I've read, a very small amount of the N2 can dissolve in the beer, but it is mostly for pushing. You can carbonate with plain CO2 or with the blend, I've done both and both work.
If you want 1.2 volumes then set the CO2 pressure based on the temp of the beer and shake it a lot. Go rest, then shake it some more. Shake it as much as you can over the next 30 hours. You can hook it up to the beer gas right before serving. Do you know how to determine your beer gas serving pressure?