Filming a 40 min play - sync problems?

I'm going to film a 40 min play with my 5D. And as many know it only allows me 30 min of continuous recording.
I'll be using two cameras, and after about 15 minutes I will start recording with my second camera. And when I'm getting close to 30 minutes, I will turn on and off recording on the main camera. I tried that last year, but synching went to hell.

How can I film a 40 min play with dslr and external recorder?

I would let the external audio recorder go non-stop, recording the entire event. The only issue is when your 1st camera reaches it's unfortunate recording limit. However, if you have 2 cameras this is really no problem. Just make sure camera 2 is recording when you stop and then continue recording (a 2nd clip) on camera 1. That way you'll have total coverage, and in post you can just select the external audio track and sync it with the 2 clips from camera 1 and the 1-2 clips from camera 2. Should be a breeze --just MAKE SURE both cameras are recording audio at a decent level so Final Cut Pro X can use it to properly sync.

Thanks a lot!

I thought I did that last time I was filming a play. Back then, the sync didn't work. I'll better do this right this year.
Thank you for you help, Gabriel!