All I can say is... If you played The Witcher 3 (great game), and went with both Yennifer and Triss, and they find out about it, well they are not too happy about it.
But the game goes on, and you still have missions with both.
It's tricky for sure, but in the end it really depends on the LI, each has their own personality.
Some are more open minded, other old fashioned.
At the beginning phase where you "fool around" and explore possibilities you can date whoever you'd like, its only an issue once it starts to get more serious, deeper into the game.