I will leave my 2005 mercedes SL500 in my garage for 4 months. I have battery tenders. Should I put one on both batteries, or if I put one on the starter battery will that keep both charged?

If the batteries are isolated from each other then you would want two tenders.

I have two tenders, but do not now if the Batteries are isolated.

I doubt it would hurt anything to use two in any case as long as you don't reverse polarity. If the batteries are isolated then only one might get a charge if you use one. Does your owners manual have any information on this? If you only used one I would use it on the starting battery.

Could the answer be to remove the starter battery and put a tender on it outside the vehicle, and put another tender on the accesory battery in the vehicle?

That would also work.

Thanks for the help Ful_of_Regrets.

or just take the pos + cable off of both and leave the battery/s in the car........ the other thing maybe jack up the car some to get some weight off the tires/suspension...put a box of baking soda inside the car also...