I do not know if this is already common knowlegde, but I may have found a way without mods to get two romances going without problems.
In my current playthrough, I have both Jaheira and Viconia in my team. All is going well, but knowing how the game works and how multiple romances will clash at some point after certain conditions are met, I made some testing because I was curious if this could be circumvented without cheating - and because it would be nice both to have a sensual drow mistress and a fiery druid, which strokes Charnames Oedidus complex, as lovers at the same time. Hey don't judge me. We all have thought the same :-)How to circumvent the second "choose me or her discussion trigger"
Because Viconia and Jaheira second "choose me or her discussion" is so late in Viconias romance, it is often possible to circumvent this trigger, when Jaheira leaves for the harper hold, after Dermin has forced her to attend it. Jaheira will not return to the team without you picking her up.
If Jaheira is not in the team when Viconias second "chose me or her discussion" is brought up, both romances will continue as normal, as the second "choose me or her discussion" love talk does not trigger. The game will not terminate the other romance.
The only prerequisite/condition is that you pick Virconia up a bit later, so the Jaheira romance can be well underway (i picked her up when I hit "summoned by Galverey quest"), This way you will very likely hit Jaheria's "is summoned to the Harper hold quest by Dermin" a long time before you hit Viconias second "chose me or her discussion".
When Jaheira leaves for the Harper hold, you can catch up on Viconias romance. After you have triggered Viconias lovetalk variable that normally would trigger the second "choose me or her discussion trigger" if Jaheria was there, pick Jaheira up at the Harper hold and continue playing.
Voila, you now have two romances going at the same time. I do not know how this will play out in Throne of Bhaal at the temple of Mir confrontation. Maybe the master shade will get a case of immediate split personality disorder. Oh well.