ACR + battery tender = bad idea?

I've got a two battery setup including a Blue Sea ACR. Small stereo + little sitting around = no need for shore power through the summer.

Is it a bad idea to hook up a battery tender in the winter without disconnecting the ACR? It seems that one battery would hit 13V, then the ACR would close... of course a tender can not keep up with the current needs, and voltage would drop and maybe open the ACR. Seems like a lot of cycling and stress on the battery that was charged first.

thoughts?