Specifically, I'm trying to work with King's Quest 2 booter version (it's subtly different from the DOS version). DOSBox is running in Tandy mode.

What I have determined (and please correct me if I'm wrong on this), is that the game has two disks, and requires a blank disk for saving. Disk 1 is a booter, and disk 2 contains whatever game data doesn't fit on disk 1. The game must boot from the A drive, and the save disk must be in the B drive. So if you were to play the game with an actual Tandy, you would put Disk 1 in the A drive, the blank disk in the B drive, and you would swap the disks in the A drive frequently, but never swap the B drive.

In DOSBox, I have the following images:
KQ210W-1.IMG
KQ210W-2.IMG
BLANK.IMG

With this command:
BOOT KQ210W-1.IMG KQ210W-2.IMG BLANK.IMG

I now have the following three floppy states available, cyclable with CTRL+F4:
A: KQ210W-1.IMG
B: KQ210W-2.IMG

A: KQ210W-2.IMG
B: BLANK.IMG

A: BLANK.IMG
B: KQ210W-1.IMG

This works, but is a bit inconvenient, and if I accidentally have BLANK in the A drive, bad things happen. I'd rather have is CTRL+F4 swap the A drive, and leave the B drive alone. There would be only two states:

A: KQ210W-1.IMG
B: BLANK.IMG

A: KQ210W-2.IMG
B: BLANK.IMG

Is this possible? I've tried messing around a bit with a combination of IMGMOUNT and BOOT, but I can't get it to work quite the way I want.

The other thing I've done is this:
BOOT KQ210W-1.IMG BLANK.IMG KQ210W-2.IMG BLANK.IMG

With this, I have to press CTRL+F4 twice to change the A drive while leaving the B drive alone, which is better, but it's still less intuitive than what I want.