I made a large purchase that had to be paid with multiple payment sources.

I can find lots of information for splitting a single payment into multiple expenses, but how does one go about recording the opposite case? For example, a $10,000 expense that was paid $6,000 from a checking account and $4,000 on a credit card account.

ANSWER:

Start with entering a Bill for this purchase of $10,000

From the home page screen > Create + sign > vendors > bill
Fill out bill form. See attached photo 1 for reference.
.

Then use Pay Bills, and pay by using the bank checking account for that $6,000 part. Then, Pay bills, and do the credit card account for $4,000 part.
From the home page screen > Create + sign > vendors > pay bills
See attached photo 2 for manual check payment reference.
See attached photo 3 for credit card payment reference.