Did I Ruin My Battery?

I was using two batteries with my UPS, connected parallel with UPS (HOMAGE). Due to over charging problem in First battery, I simply disconnected / plug out -ve terminal of this battery to protect it from over-charging. Other battery remained connected with UPS. The First battery become dead in such arrangement in Two months.

Is it due to the problem that First battery's +VE terminal remained connected during the chargeing of 2nd battery and +ve terminal of First Battery received current while the -VE terminal was pluged out which ruined my First battery?

Alsi is there a way for revival of battery in such case?