To answer the question: No, Bitpay doesn't support split payment. It is (at its' current state) designed to provide a solution for merchants, who don't want to bother with technical stuff like this and not act as a full payment-processing-service. That payment-processing-service is the Bitcoin-protocoll itself already. And sites like yours should build directly on top of that and not on top of some additional layer that may act as current credit-card-companies.
I don't know if bitpay supports this but you could always run bitcoind on a server and add a few lines of code to it. Someone will always be able to do it for you.
then ... why he would need BitPay if he takes
BTC directly?
He doesn't.
Bitpay's key-feature is offering Bitcoin payment with the posibility of getting paid out in fiat.
With Bitpay you can only have one payout-address of a kind registered at a time. (so one $-bank-account, one €-account, 1 BTC-address, ect...) - for your site I wouldn't suggest you to use Bitpay anyways, because to get the money/coins to the campaign creators, it would have to go through one of your own accounts anyways if you used Bitpay.
That being said, if you plan a BTC-only-site, it'll be easier (and probably cheaper) for you to setup your own bitcoind.
I'm interested in your project though, since campaigns can take 30-40 days, considering the volatility, this could(in extreme cases) mean either half or double the value initially requested.
