In most cases, mapping software knows no borders but the maps do ;-)
As for the maps, have a look at my comercial website - www.canadianmaps.ca I have topos for most of Canada. They are calibrated for TTQV software. They can be used with other mapping software but they would have to be individually calibrated (georeferenced). Now, even that TTQV can be used to calibrate "old maps", it has no function to display two maps overlayd on top of each other - only side by side. For that, you could try GlobalMapper... also avaliable from my site. In that, you can display several layers of maps and data and set transparency levels. Once the map is calibrated, the software will automatically stretch / shrink / whatever to display it in projection of your choice. Works really well but it's not cheap. Well, the trial is free so you can see if it does what you want.
Sorry if it sounds like a sales pitch. I simply don't know which other software can stretch / re-project maps but I'm sure that there is some out there that does it.
Andrew