2 cars! And lots of bathrooms!
I think it's a good idea if you have an organized plan with what parks for what days. Know what the plan is for each day so that you don't spend an hour deciding where to go! Ask them what they want to do, ask your family. Figure out things/meals you can all do to together. I use a simple spreadsheet & put park hours & stuff on it.
It can be a good idea to start off together in the morning if your touring styles match up. Then split up for the afternoons & meet up again for dinner.
Another way to split up might be older boys & younger ones.
I think the most important thing is to talk about it beforehand and see what the expectations are. When we went with my MIL, SIL and neice/nephew they were happy to have me lead them around. I knew what they wanted to see and the best way to get it done at a busy time (july). We split up a few times after touring in the morning & one day MIL & I went shopping while DH, SIL & the tweens did the big rides!
We went to Universal last year with BIL & SIL & their son. They are very relaxed about everything! I like to get up early & go when the parks open. They roll out of the house around 10-11. It was a little hard for me at first, but I didn't stress and just had fun. We didn't have any pressure to see or do anything, because our kids were so little and none of us are big roller coaster fans. It was really fun to hang out and not worry.