The topic does seem to come up often enough that there seems to be sufficient interest to warrant manufacturer attention. I'm hoping that 300Bucks has success with marketing the 2-blade to Buck. The Case (uncredited Bose) Wharncliffe trappers and Hawbaker muskrats like the Case/Bose Muskrat are similar 2-spring knives with a clip and straight blade. GEC made the Conductor pattern with a clip and coping blade on a single spring. Using a single spring makes a much more streamlined knife. I hope to have a 2-blade 1-spring Sowbelly custom made.
Going through the old catalogs, you'll actually find lots of examples of knives with either a drop point or clip main and straight secondary. Here's a beautiful example of an old sleaveboard with a clip and coping secondary. ...even has a lockback.