I think that was called "start assist" mode, but I don't like it because (as you may have found out), your start bank can go bad without you realizing it as the house bank "silently" takes over. Ugh! I'm in the process of re-wiring my rig and am not going to have that "feature" in my new plan (my Surepower 1315 is on the recall list, so all the more reason.)
I'm actually debating between installing the Blue Sea 7622 I have, or just going with a manual switch. I'm leaning toward the manual switch for a few reasons (perhaps you could say "quirks
1) I'm used to remembering to do things like this from boating.
2) I like the idea of my default arrangement being that the start and house banks have nothing to do with each other.
3) I can the choose to "combine" the banks if my house bank needs charging and I am driving for awhile.
4) I watch my house bank like a hawk when I'm parked (battery monitors), so even if there were a time I forgot and left them combined, I think I should still have enough voltage to start because I don't draw the house bank below 50% (not that this would be the way I'd intend to run things, but it's good to have a workable plan if I forget to un-combine).
5) I have ordered an Anti-Gravity jump start pack for "situations." A friend has one and we started his V-10 (what I have) the other day three times with it and it still had almost all of its charge (granted it was not cold weather). Pretty amazing (and will charge electronic "devices" too).
I may decide to put in the 7622, but my current leaning is toward just a manual switch. The way I have my battery bank set up I would be able to reach the combine switch from the driver's seat (would be on forward facing side of gaucho). I would plan to leave it un-combined except for specific driving days when I wanted to charge the house bank from the alternator.
Feel free to shoot my idea down, anyone - I may be missing something. And I do have the 7622...