I've run large groups (up to 9 people) before, and the first rule of thumb is that some players will always drop out. This isn't a reflection on you or your campaign, but a simple fact of life - people get busy, life happens, etc. So you're likely to end up with fewer than 9 regular players. (My first 9-PC group dropped to five regular players, then added a sixth much later; my second has dropped from 9 to 7 and two more know they will have to leave in the spring.)
The second rule of thumb is to keep things focused and moving. With up to 9 PCs, it's very easy for the game to get bogged down as everyone wants to go off and do their own thing. Make sure you use situations which allow you some control over what the PCs are doing. Which isn't to say railroad them - just that open-world/sandbox campaigns are VERY difficult to run with large groups, and you should consider focusing on strong plot drivers that give the group something around which to unite.
The third rule of thumb is to find ways to keep combat interesting. This means balancing your encounters properly, yes (although how you do that depends heavily on your group, i.e., lots of strikers? Mostly defenders? Players who would prefer traps and puzzles in the middle of their combats? Players who just want to blast orcs in the face? etc). But it also means finding ways to trim off the boring bits. For most encounters, especially if they've already run on for several rounds and most PCs are down to at-will powers and/or it's very clear that the PCs will win the fight, I will ask the group if they want to just kill it. This ends the fight as if we'd played out the whole combat and doesn't cost them anything; it just means we as players don't have to sit through the last however many rounds of the PCs pounding on the remaining monster(s).
Regarding your idea of splitting the group, I'd be cautious with this. Splitting the group is likely to end up being a complex proposition, and whether it works would depend largely on how much time you're willing to invest in both groups and how good you are at juggling them. Consider the following points:
- If you run two split parties during the same session, then for half the session, half your players will be sitting around doing nothing. Boring!
- If you run two split parties on alternating sessions, your players' progression will suffer as each group is only progressing half the time.
- If Party A does something that will affect Party B, you have to be prepared to integrate that into whatever you had planned for Party B.
- If Party A does something that requires Party B's immediate input (e.g., decides to ambush the group because B has something A wants), then the game grinds to a halt until you can get both groups playing at the same time.
Which is not to say it's impossible, but it will definitely take a LOT of effort on your part.
My personal preference has been to just manage the large group as one. Especially considering that you're likely to lose some number of players over the first few months, this isn't as daunting a prospect as it sounds.
And finally, remember to communicate with your players! They want the game to be fun just as much as you do, so communicating up front that this will be a large group and you'd appreciate any help you can get (such as avoiding off-topic talk during important moments, players taking over minor duties during combat such as initiative tracking, etc) will go a long way.