I always let the CSC cashier know that there will be split payments of $XXX per swipe. This has two very important benefits.

Some WM CSC reps only allow two swipes (although the store will allow 4 from my experience). This allows me to back out of the transaction before any transaction occurs. I simply tell the person that I cannot pay for the MO with two swipes and cancel the transaction. I do walk away empty-handed but know not to use that specific person in the future.

The second benefit is that the cashier knows what I am going to do. That makes for a smoother transaction, limits questions and speeds up the process, a big plus when there is a line behind me.