Offroad-Express

I have searched and not found an answer, so....

One of my my batteries in my diesel 80 series is on the way out.
My question is, do I need to replace these batteries as a pair like in a full time 24 volt truck (batteries wired in series), or will the older battery have little or no effect on the new battery being that majority of the time (apart from starting)the batteries are in parallel?

Cheers

Jase.

I had this recently.... Replaced the one that was knackered as tested by auto sparky.... all good... until 2 weeks later, had to replace the other one as it kept draining the brand new one.

Has been all good since though...

Jay