IF the installer of the AGM specific did not do it I'd "Cut the cord" to the 7100's charger module.. here is how to do that.
Open the power distribution panel, Find the wire that goes from a circuit breaker most likely labeled CONV to the back of the panel. remove and tape it off (or just turn the breaker off and tape it).. That way if the external charger ever fails you can easily restore the 7100s
The 7100 is a low end single stage converter, In fact I'd call it a power supply, It is not really that good at charging batteries since it is a single stage.. However running it in parallel with the other charger very likely will do no damage.. May slow charging a bit is all by "Tricking" the AGM charger into thinking the batteries are fuller than they are.