Here are two ways to do what you want.

You want to use components A and B to make finished good C. You want to add $10 per unit additional cost to C, of which $5 gets credited to expense acct LAB REWIND and $5 gets credited to expense acct LAB MACHINE.

Scenario 1: This is the only finished good that has a BOM with additional costs, or all finished goods with additional costs have those costs split in the same percentage (in this case 50/50). You can do the split by setting up the Assembly Cost Credit account of the finished good to go to another GL account that is defined as an Automatic Reallocation account, and do the split in the GL.

Scenario 2: More than one finished good needs to have multiple additional costs, and the percentage varies. You create two BOMs. The first BOM uses components A and B and $5 of additional cost, and the finished good is a fake item AB. This item is linked to a new IC Control Account Set that has as its Assembly Cost Credit account the GL account LAB REWIND. The second BOM uses item AB as a component and $5 of additional cost, and the finished good is item C. Item C is linked to an IC Control Account Set that has as its Assembly Cost Credit account the GL account LAB MACHINE.

Scenario 1 is a longshot, but if it works for you, there is no additional work on your part. Scenario 2 is doable, but it requires you to do two assemblies each time you want to make item C.

Steve Schwartz