That stumped me at first too.
The way I understand it is you can only display two of them in your campsite at once. But once you've built a tent, then the effects of it remain. So you can go ahead and build over top of an existing tent to gain the effects of a new one, then put the previous tent back afterwards if you prefer its design.
Or at least I'm pretty sure that how it works from my experience if anyone wants to chime in to confirm.