I used to like battery tender, till I found out their warranty was garbage, and of course that fire the one member had.

I currently use shumacker and ctek.

The ctek one is for agm batteries and has the desulfating thing built in. no idea if it works well or not.
It doesn't seem to save batteries, or make them last any longer really. Still been replacing them every 5 years or so just like normal.

But it does prevent them being dead after sitting for a while. or doing lots of short hops.

As for charging multiple batteries it can, it will just be slower.