Recommend separate exhausts
I would advise against tying the two together. You may introduce the possibility of forcing water from the main engine exhaust into the genset engine (if it were not running) or vice versa. I just finished a self-install of a genset. I used a 1.5" bronze mushroom through-hull for the exhaust. Cost about $50 at WM. No big deal to install so long as you're out of the water.