It MAY be OK, but I wouldn't do it even if you have a make-before-break battery switch. The potential of a malfunction -- human or mechanical/electrical -- is always there, and the result could be catastrophic for your alternator's diodes.
It makes MUCH better sense in your situation to:
1. always leave the battery switch in the house battery position;
2. always charge the house batteries; and
3. use a voltage follower device like an EchoCharge or DuoCharge (or other) to keep the start battery topped up.
This is a relatively inexpensive upgrade (about $130 for parts), and completely avoids the need to switch anything. It's totally automatic.
If you really wanna do it right, in addition to the above you could install a simple ON-OFF battery switch in the starting circuit, and a hefty fuse (not required by ABYC standards, but a VERY GOOD idea). A good inexpensive switch is the Blue Sea Systems #6006 -- about $20.
Last edited by btrayfors; 07-24-2011 at 05:49 PM.