That would be nice, but it doesn't exist. Garmin's map format is proprietary and has never been officially published. The maps on this site were made by third party tools that have reverse-engineered Garmin's format. If Garmin wanted to be nasty, they could probably prevent publication of these maps, but since they add value to their products, they choose to look the other way.
So there isn't much incentive for someone to develop a commercial Garmin-compatible desktop mapping program due to intellectual property issues, plus that fact that most people find Basecamp adequate and it is free.
Mapsource is your only alternative, and some people still prefer it over Basecamp. But since it is discontinued, it is not fully compatible with the newer Garmin devices (will not recognize tracks, routes and waypoints on the GPS because they are stored differently than the old models).
There are various ways that you can view more than one map at a time in Mapsource or Basecamp if you know what you are doing, but it is kludgy. For various reasons, it was decided that this would not be discussed in the forums here. But if you contact jbensman, the MyTrails author, I suspect he could point you in the right direction.
Also note that Basecamp CAN show two maps as long as one of them is raster-based (such as Birdseye or a a Garmin Custom .kmz map). So you could create your own .kmz topo map, or purchase Birdseye Topo ($30) and use it as a background for the trail map.