Mark, you're very welcome.
If you can get behind the wall, you can mount a two-gang box behind the switch-box, relocate the wires to the new box, and use short pieces of two- or three-conductor Romex® cable to connect to the dimmers.
That way, the only wires in the box with the dimmers would be those that connect to the dimmers themselves; no unneccesary neutrals, fed-through cables, etc.
Another psooibility is that a replacement, old-work box might be deeper, thus larger, than the existing box. Most boxes are marked somewhere inside with the internal volume they provide.
Larry
www.fineelectricco.com