Currently it is possible in Boxer to swap physical CDs by ejecting one from your Mac's DVD drive and inserting the next one, allowing you to play multi-CD games.
There should be a way to swap non-physical CD and floppy-disk drives too, whether they're stored as ISO images or as .cdrom/.floppy folders.
The proposal is to create drive 'stacks', whereby a single DOS drive letter can be given multiple image/folder source volumes, of which only one is active at a time.
=== The UI for stacked drives === These would appear in the Drives panel as a stack of icons, much like stacks in the OS X Dock. Clicking on the drive would cycle the drive to the next source image/folder in the stack, making it active in DOS and putting its icon at the front of the stack.
There should also be a keyboard hotkey to cycle all drive stacks, so that a user does not need to drop out of fullscreen mode in order to swap discs.
Images or folders could be stacked onto an existing drive by dragging them onto that drive in the drives panel, instead of into empty space. Drag-dropping multiple images/folders into empty space should create a new stacked drive from them, instead of mounting them as individual drives.
The Add Drive… panel should be modified to let you choose an existing drive on which to create a stack, and to support the selection of multiple items.
Later, the drives panel UI could be extended to add a popup displaying all the drives in the selected stack, allowing them to be drag-rearranged to change their order or pulled out individually to discard them. This popup would resemble the grid appearance for stacks in the OS X Dock.
=== Drive stacks in gameboxes ===
Boxer should create these drive stacks automatically when it finds more than one floppy/CD volume bundled in a gamebox. Any image or mountable folder in a gamebox //which doesn't have a specific drive letter of its own// should go into a stack with any previous image/folder of the same drive type.
A user can make the bundled volumes mount on separate drives (the old behaviour) by giving them explicit drive letters in their filenames. A user can likewise assign a set of volumes to mount as a stack on a particular drive by giving them all the same drive letter.
This backwards-incompatible change to Boxer's default behaviour is justifiable since very few DOS games recognise or take advantage of multiple CD drives side-by-side, and so the old behaviour was largely useless. Floppy-based games can often utilise both A: and B:, but most floppy installers expect the game to be installed only from a single drive.