I think it is being assumed here that you have these batteries set up, or switched, to run in parallel, meaning you have the amp hours of both available. Is that true? That will be important when figuring if you are going to be drawing them down by 50% etc.
There are a lot of threads here that cover this topic, if you can find them. Some good reading in there.
2000 Beneteau 331
Northern Chesapeake Bay
Ambition is a poor excuse for not having enough sense to be lazy ~ Steven Wright