6 volt charging

[hankpac]

34 RLS, 2008
I have two 6 volt batteries that I hook up for 12 volts.
I pull these from the RV to put in the utility room (warmer but not hot). It is down to 20 below at times outside here.
I want to put them on a Battery Tender, but when I hook up exactly like it was in the rig, the tender flashes "wrong hook up".
I have a positive lead from the tender hooked to positive, on one battery (batt 1), the negative lead hooked to the negative pole on the other battery Batt 2), and a lead going from the negative on batt 1 to the positive on batt 2.
I wrote to Interstate, and they sent me a chart exactly like I described above. I have a 6 volt charger too, but that means hooking them up one at a time, and that is just a hassle.
They are 90% charged right now.
Thanks

[Herk7769]

As long as your charger is hooked up like you describe your 12 volt battery tender should work. A "wrong hookup" error means one is backwards.