I had several times that parents told me that they are paying for their son. Or vice versa. So I think it would be a good idea that you should be able to apply a payment from one donor to the pledge of another Donor
D
Dovid Schmidt
Also when someone in a Beis Hamedrash gives a big check and asks that it should pay for his own pledge and someone else. You should be able to split the payment to a few peoples pledges.