I am fully aware of what they are. Both play on the right-hand side of central midfield. Rabiot's best position is as a #8, albeit it seems he's the only one of the three, who has no problem playing on either side of central midfied. He doesn't want to play as the holding midfielder though, even if he's still better than most there. Pogba and Aouar play almost exclusively on the left-hand side of midfield. Several high profile managers care about whether a centre-back needs to played on the left or right-hand side of the central defence. It makes a difference to their performances. Tielemans has played almost every game on the right-hand side, wether he has lined up for Monaco, Belgium or Leicester, the same as NDombele. This wouldn't make sense. It would be the same stuff we did before, when we had Rooney and signed Kagawa, Mata, di Maria and Mkhitaryan. Their best position was very similar to that of Rooney and we've had a problem on the right wing ever since that stupid strategy.
I've got no problem with the players, in fact both Rabiot and NDombele would be excellent and they would work together IMO. But Tielemans and NDombele wouldn't work, or rather there is no indication that either is as comfortable on the left-hand side to accommodate the other staying in their favoured position.
Yes, that's his best position, but how would that work with NDombele given what I've written above? NDombele likes to get forward himself, if he was Tousart behind him or Aouar dropping deeper.