Connecting two 90mm tank overflows into 100mm stormwater

I have just had a 4000g water tank delivered. The tank comes with two 90mm overflows (to match the two inlets). I was planning on connecting both overflows to a 100mm stormwater run-off pipe instead of having to run two separate 90mm stormwater pipes (about a 20 metre run). Would this size pipe be sufficient to prevent water backup in a storm?

No probs, just remember that most home's stormwater discharge point is 100mm and that covers your whole house!

you could cap one of them off, because they are the same height and are made for 2 for different tank positions. so they arent exactly there so both can be used