See, it's strange, because I could have sworn using two credit cards was an option online (during checkout.. there was a link to "Use Two Credit Cards") when I bought something a few months ago, but when I went to buy my MBP just last week, I couldn't find it. I thought perhaps it was because I was going through the developer store, but couldn't understand why.
The only suggestion I have would be to maybe call Apple.. perhaps when placing an order over the phone they could process two cards?
Oh, and to dextertangocci: using credit cards doesn't necessarily mean you're actually paying with credit. While of course it could, in my case I wanted to pay out of two debit accounts (paypal and my bank account).