The two are very different, and would coexist well, as far as I know (I've installed both of them on my BG2, without a hitch).
The Flirt Pack adds a little spice to the Bioware romances (and allows you to add interactions between Jaheira/Viconia/Aerie/Anomen and Solaufein if you have the Solaufein mod installed). With this mod, you are able to kiss, tickle, hug, flash, bathe with, etc. your heart's desire.
With Romantic Encounters, things are a bit different. You may still pursue a Bioware romance, but you now have the option to flirt or sleep your way across Faerun! There are quite a number of encounters available to you (I believe the ReadMe has more details and a complete list, if memory serves), with varying degrees (from PG-13 to Mature, as ratings go). The neat little tidbit is that your Bioware partner won't always stay quiet about it (and if I'm not mistaken, I don't think the Encounters can break the romance). Unfortunately, RE is hosted over at the Gibberlings Three forums, and they're down for now. But they should be up soon, so just be patient.
All in all, it depends what you expect in a mod, but these will certainly liven up your love life in the Realms!