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Clean your battery connections. The stater solenoid and starter connections too.
Have the batteries load tested. They must be check individually. If one battery is bad you need to replace both at the same time IMO. Have your charging system checked too.
Check all battery cable connections by disconnecting and cleaning and re-connecting. Load test both batteries individually while they are disconnected. If they test out good and the cleaning service doesn't solve your problem it's probably your starter. If you put the batteries overnight on a good 10 amp charger and it starts up fine in the am, then it's probably one of your batteries or maybe your charging system is not up to snuff. It's probably going to end up being your starter but you should do the above-mentioned stuff anyway because reduced power to a new starter motor will quickly turn it into a weak used starter motor.
Clean up those connections, mine did this same thing and the batteries were about 1 year old. There is a red spray that you can put on the terminals to help prevent this from happening again.