I really don't see how they are "very different". They're both perfect for the trequartista role, both mainly rely on their vision, high football IQ and passing and both lack an offensive game to play higher up the pitch. In my opinion you can't have both in your starting line up when using a 4-3-3 formation, when you go to a 4-2-3-1 it could be possible.
Busquets is much more of a true DM than Frenkie is, who's more of a CM these days. Somehow people view Frenkie as a DM all of a sudden. Frenkie is an CAM of origin, in fact, but Bosz and Ten Hag have both used him in a lower role since he is so confident on the ball. Frenkie's game relies on his dribbles first and foremost. He lures the opposition to pressure him, creating space for those teammates in front of him. Busquets rarely goes on any adventure and plays a rather safe game these days. I don't think they are similar players at all. Not to mention Frenkie has a lot of offensive upside, but it won't come out if he's used as a DM.
I think a Busquets - Frenkie midfield could very well work..
---------------- Griezmann -----------------
Suarez -------------------------- Messi -----
------------------ Rakitic --------------------
--------- De Jong ----- Busquets -----------
.. just not the midfield I'd prefer.