I can confirm that if you download to one internal card, then switch it to the external slot, the maps app finds the maps still. Thus, just put the card you want the maps on into the internal slot for download.

Also, it seems you need somewhat more than the space the map takes (maybe 2x as much?) free before the download and install will succeed. I tried putting the USA east maps onto the device with about 900 MB free on the internal card, and it kept failing at the install point. Not 100% sure that was the problem, but it seems to be an issue.