pay with two debit cards for one purchase

Apple doesn't usually accept debit cards that you have bought in a local store and wasn't issued to you by a bank in which you have a checking or savings account from which the card draws funds.

Late 2012 Mac mini, OS X Yosemite 10.10.3; Apple Watch, 38 mm silver AL, Watch OS 1.0.1; iPad Air 2 & iPhone 6+, iOS 8.3

Apple does allow you to use at least two different bank cards when making a purchase. I did it with a debit card and a credit card when I bought my  Watch.