We bought an old farm house with no indoor bathroom. We want to put one on each floor but where the pipe going into the septic tank in the basement is about 16 inches from the outside wall and that is about where we want the toilets to go on the main and 2nd floor. Can we offset the vent stack? Or do we offset the toilets? Can someone steer us in the right direction?

Offset is fine but make sure all horizontal runs pitch back toward the main drain so condensation will not build in the stack

Bob nailed it! You can have as many bends as you want in a vent stack as long as you maintain slope back to the drain, Good luck, Tom

Thanks guys, much appreciated. We thought so but easier to get the real answer from someone who has nothing to gain and just wants to help someone. If you ever need help with raising pigs or chickens, give me a call. :)

Just curious as an amateur. How do you maintain pitch? Use 45 degree elbows?

Who said we'd use a 45, I use 60s when in a pinch like this.