You'll need to have a setup that allows for splitting a payment. Typically, in a situation like this, I would suggest that you have a 3rd table that would act as a Child table to the other 2 tables, but that may not work for your use case. Instead, you may want to still create a new table - we'll call it Checks or possibly Transactions (depending on if you accept multiple forms of payment). This could be a child of Customers and a Parent to Payments. This would allow you to maintain your existing structure and not have to rebuild the current process significantly.
------------------------------
Blake Harrison
bharrison@datablender.io
DataBlender - Quickbase Solution Provider
Atlanta GA
404.800.1702 /
http://datablender.io/------------------------------