You could try swapping the batteries from side to side when you install the converter in step-down mode. If it is not significantly weakened, you may well be fine.
As Stone says, it is "best" to start with two identical batteries.
I've heard tell that you can charge both batteries hot, which will balance them. Shhot me a note at
gbowie@uniserve.com and I will e-mail you an info sheet on the converter I have put together.
Regards
gb