I doubt it would be much different than taking MDMA both times, except the 6APB has a longer duration of action and is a little less stimulative. If anything I'd guess you would probably get a better overall effect because the substances are not exactly the same. I'd have extra on hand of both just to make sure if the 9 PM dose didn't get you right you can get yourself right by 10 PM. I think the sound of 3 PM dosing and 9 PM dosing are pretty staggered. Ideally for optimum synergy between 2 psychs you probably wouldn't wait so long between using them. But it's all about what you're after. And 100 mg MDMA might not feel like very much after you've already done 6APB 6 hours earlier. Personally at these hot summer festivals I wait until the sun starts to go down before doing anything, which is around 8 PM around here. You might find the heat a bit sobering, hence, I'd suggest maybe doing 6APB at 7:30 PM and MDMA at 9 PM.
But uhh something about festival and can't get enough uhh... maybe you should just wait until you get there and see what is in stock? ;P