I suppose that would depend on the location of the 2nd battery. I put my 2nd battery opposite the 1st battery in the stern so it was a shorter run to the motor than tying to the negative post of the first.. but no reason you can't.
May want to try another caliper for comparison. If you have access to a lathe you could turn it down but you couldn't add to the first stage. That said I think you could still get accurate grease readings.