I just play the game too much
This particular limit I observed at launch when I was romancing Karlach and accidentally initiated one with Gale as well without even realizing it while creating the Weave, so at the camp celebration everyone treated me like a complete stranger because I was already "taken".
Really rubbed me the wrong way because I really wanted Lae'zel to be my second, so did some testing and that's how I found it was limited to just two. Which is also how I found out that triggering two romances too quickly will lock the player out of all camp scenes which may initiate a romance, but are not even strictly for it.
For example Gale's Weave scene is a romance initiator, but it also has tons of in-depth lore about him as a person. If you romance two companions before getting this scene, you will miss out on it permanently because of the limit. So to avoid this I always leave my 2nd romance open for as long as I can so I don't miss any scenes.
