I know how to perform a parasitic draw test. Thing is, I've never done one on a truck with two batteries. This seems simple to me. Seems the answer would be yes. But since I have never done this, I should ask. You remove BOTH negative cables, right? And isolate the one that you are not working with. Also, I can't see that it would make any difference, but... Is one of the batteries preferable to the other, for the test?