It's absolutely possible. More players just means a higher potential volume of posts to read through and a likely higher amount of people requesting things of you as the GM. As long as you have the free time to spare for the reading and doing GM stuff, there shouldn't be much of a problem. Honestly I think the thing that'll be hardest about doing a 16 person RP is actually getting 16 interested people to join.

However, if you're not feeling 100% confident of your ability to do so, the easiest way to simplify things for yourself while maintaining the high player number would be to get a co-GM. You could focus on GMing one group of 8, and the co-GM could focus on the other. Or you could get two co-GMs, one to co-GM for each side while you do the overall GMing stuff. Either way, that'd make it less crazy for you to manage and thus increase the chances of it running successfully.