You would need to split the Bill, because first, the Bill is already in One AP account. The entire Bill balance is in that account. Never Pay from a different AP, because that will leave the unpaid  bill is one AP and the Negative AP in the other account.

It seems what you really need is Class Tracking, not more AP accounts. Open Help and read about Class tracking. You avoid micro-managing by more accounts, if you use Class Tracking as the cross-reference, instead.

Oh, here is your error: "I also have my Piano Lease account, which is the A/P account with the loan balance where my principal will be deducted from."

That Total Balance is not AP; that is a different type of Liability. That is either Short Term (one year) or Long Term Debt. That is not AP. AP = current and revolving Purchase Balances that you did not yet pay for. Example: You are allowed to ship Music whenever you want it, and have to pay them Monthly = enter a Bill with each music delivery. Then you Pay Bills and select everything for the cycle that you are paying.

And Credit Card is not AP, either. Mortgage is not AP. These are all different types of Liability, not AP.

In regards to the above questions...how to pay two bills from two different A/P accounts on one check to same vendor....how do you do this?

@bsff

When you enter a bill, that Bill is assigned to an AP account. You cannot put one bill into one AP account, then try to pay it as multiple AP accounts, because it doesn't exist as a balance in the Other AP account.

If a bill needs to be split into two AP accounts, by definition this is Two Bills. Or, you use Class tracking, not micro-managing more accounts.