We 12 x 6V AGM batteries in our house bank and one 12v AGM battery as the starting battery.
AGM batteries as I understand it, can be used as both start and deepcycle. We have had this setup for nearly 5 years now without problems.
One thing we did do on the house bank was to install battery balancers to ensure each battery in the bank gets fully charged. Because of small differences in internal resistance, with 4 6v batteries in a series chain charged as a 24v system, the voltage across each battery will be different and this will mean that some batteries are continually undercharged...and they die.
The same applies to 2 x 12V batteries in series but perhaps not so critical. If you have a 2 x 12v system I would suggest at least tying the middle + - links together in each set of 2 x 12. and best, do that and install a battery balancer to keep that mid voltage at exactly half whatever the charging voltage happens to be.
AGM batteries are also virtually maintenance free and have a higher charge acceptance rate than flooded.
SV Elyse SM437