I'm currently starting a track of accumulating around 3500 miles a week for some weekly business travel for the next 6-8 months.

I have 10,000 miles already with United
I have 1700 miles already with American

Should I split them up and keep going or should I stick with one? I dont know what to do.

Can I get some opinions?

You don't really give enough information in your post to arrive at an informed opinion.

Where are you originating from? What travel patterns do you have (lots of last minute changes, flying home early is possible)? Where are you flying to? Are upgrades important? Nonstops? Mileage accumulation? When you say "I have 10,000 miles already with United", does that mean miles flown this year (EQM), or miles in your account (RDM)?

Can you fly most of your flights on one of them? Probably better to shoot for status on one, likely UA, as you likely won't garner enough miles if you split them up.

For your travels, you wouldn't have enough EQMs/PQMs to qualify for the top tier in either AA or UA.
I'd stick with one program. Whether you should fly AA or UA, I can't say.

10,000 miles with United and 1700 miles with American is assumed EQM and PQM for FY13. I have 27,000 total miles with United and 10,000 miles total with American. Eventually I want to get to Million Miler in my lifetime.

I'm a traveling MC, currently flying between California and Illinois, so 3500 round trip per week. There is an equal amount of flights for both United and American going to and from at the times I want.

Not any last minute changes usually. Upgrades are important, nonstops are important.

OK, well, you're flying to a UA/AA hub (read: lots of competition for upgrades if you are not top dog elite), assuming you mean ORD (the exact airports you use is actually a factor here, since equipment varies and affects upgrade chances). 10K PQM this year on United gives you a leg up on status there, but it's not a big one.

I'd probably go for the AA Status Challenge because your travel patterns would theoretically get you to AA Executive Platinum, in my opinion the single best status achievable by mileage out there, and concentrate on AA. You'll also get more miles when you hit AA Platinum (100% RDM bonuses at 50K+, as opposed to 50% at 50K, 75% at 75K).

This assumes that you don't particularly care that oneworld is behind Star Alliance in terms of places you can redeem your miles in some parts of the world (the options for Europe aren't as good because BA, OW's primary European carrier, will stiff you for expensive fuel surcharges on TATL awards, for instance).

Stick with one until at least the 50/75K level and then go from there.

Good advice about the challenge on AA, though sometimes UA does the same (lately, looks like only for DL elites,) which would get you immediate status, unlike AA. When I was flying a great deal more, always for fun and miles, I found it helpful to have multiple top tiers, just in case one airline hade the better price or schedule, though I always found AA the best one, especially to/from/through ORD, as way too many 1k's in and around or through ORD on UA.