I don't know if there's any policy against applying funds from two Apple accounts to an in-store purchase, but I believe you would need two iPhones, each one signed into one of your accounts and the account funds added to the wallet app on each phone (iPads don't have this capability as far as I can tell). You would also need an agreeable sales associate willing to apply the funds from one, then the other, similar to how one can apply account funds toward a purchase and then a credit card for the balance, as I have done in-store.