It's simply a case of selecting some map tiles in Mapsource, selecting another map type, selecting more tiles, and so on - then uploading the resulting collection of maps to the GPS.
Only the SMC contours are usefully transparent, and will usefully appear on top, the other maps will opaquely overlay each other in arbitrary order, but that's okay because you probably only want one type of map at a time.
I don't know the 705, but on the eTrex, mapsets can be selected by selecting "setup map", moving to the "Information" tab, then pressing the menu button again to bring up a menu that allows you to bulk hide/show maps by type. Show City Navigator and Contours, hide all the rest, and away you go.
(If you're doing this, I recommend not selecting the "Names" and "Voids" tiles from the SMC contours set. The other maps will provide better positioned place names, and the voids can be confusing clutter.)