A good friend of mine has both the xEM and xNT side by side in one hand. There is a good 5-6mm between them and we have seen no performance issues. Seriously only recommend this if you have someone to install for you that has done this before as it seems tricky to get the placement right.
To further add to Amal’s point - I wouldn’t do this with two implants of the same frequency. It WILL DEFINITELY cause issues!