Never mix battery types, or ages if you want the best performance.
AGM (Glass Mat) batteries don't like to be paired up with lead-acid
batteries, and for what they cost you're better off getting two. Mine have
been going for four years now, recharging off solar panels, without any
problems.
Roger Hightower, WBCCI #4165, VAC
1975 31' Sovereign
2002 Ford F-250 PSD