You've only got about 1.7GiB all up for maps on that 2720. You can have 2 detail maps gmapprom.img and gmapsupp.img, one can be a CN and the other a Topo. No problem either with 2 of the same [but then none of the other] for that matter. The problem in wanting to use a Topo is space. gmapprom is stored in region 49 and gmapsupp is in region 10. Those regions share space, so if you've got a quite big CN you won't have much room for a topo and vice versa. Using MapUpdater to load a purchased update of your factory loaded map via download will overwrite your gmapprom. You can also do this: [Only registered and activated users can see links. ], essentially the same as here: [Only registered and activated users can see links. ]. Using MapInstall to load the whole or only some parts [tiles] of a map from the PC Install version will overwrite your gmapsupp. You can also use SendMap20 to write a gmapsupp to region 10: [Only registered and activated users can see links. ].
Unfortunately most Topo maps aren't available in a PC Install version and you're stuck only with the full image version. First determine the size of your intended topo and then look how much space you have left available on the 2720 before you go any further. As you know from the other thread about your Quest 2, there are other ways to send maps to these old 'region-only' units as well. It's also possible to combine the NT & Topo map images using [Only registered and activated users can see links. ] to then only have to send one map to the unit.