We have a customer that recently merged all their location's AP duties to one central clearing house.  Because of this change, we received a check for $1824.44 (Invoice for $1834.44 & a Credit Memo for $10.00) which is payment for two of our customers (ABC & XYZ).
 
I can fake the system with an overpayment (full amount) to ABC Company but how do I handle the Credit Memo??
 
Also, I thought I once saw a screen where I could merge two customers... was I dreaming? ???

I would enter it as if it were 2 separate checks from the separate customers for the separate amounts.

Now I understand.  You have the following:
Customer XYZ has open invoice $1834.44
Customer ABC has open Credit Memo $10
You have a single check of $1824.44.
 
1.  Enter a Credit Memo for $10 to XYZ
2.  Enter a voucher for $10 to ABC.  Post to the same GL account as the credit memo entered in item 1 so they cancel out.
3.  Enter the $1824.44 check as payment to XYZ and select the $10 credit memo along wioth the open $1834.44 invoice.
4.  Enter a $0 payment to ABC and choose the $10 Credit Memo and the $10 Voucher to clear them from open Aging.