Offroad-Express

I know this might sound like a dum question but, I have a twin battery system in my 2.7 td terrano and the "starter" battery has crapped itself. Am I safe just replacing that one or do I have to replace both?

replaced the starter(main) battery this morning, and everything went sweet as, truck back to normal. went to use it tonight and dead, no power. secondary battery hot to touch, put it on battery charger and it came up fully charged, put on the new battery and dead, is the secondary battery feeding off the new. no new electrics added. could the alternator feed to the secondary batt be stuffed?

I had a simular problem but it was the winch battery going dead, in the end I found my chassis earth bond had failed and it was doing some sort of back feed? Once I replaced the bonding cable everything was back to normal.

If a battery is getting hot to the touch it is either being overcharged and or current consumption excessive and or batteryinternal resistance incorrect .What ever the case this is not good.Batteries do not like being cooked.Sounds like you need the charging/electrical system checked over by an expert.