We all know the Yankees doesn't have the prospects to trade for Soto and Castillo but the true is the Yankees could do both or at least better position themselves to do so but only if they trade other surplus pieces currently in the major team for more prospects.
One trade candidate could be Gleyber Torres (his trade value will never will be higher than this year, especially because he still have 2 more ARB years before FA). The Yankees could trade him to the Mariners that could improve at 2B with his 124 WRC+ and replace Adam Frazier and his 79 WRC+.
Another trade candidate could be Jameson Taillon because he probably would not start in the playoffs, if we assume that Cole, Cortes and a healthy Severino will be better options to do so. He will be a FA after this year also and I don't think the Yankees will extend him neither or give him the QO. The Cardinals are desperate for starting pitching and could trade a couple of good prospects for him and be their third option behind Mikolas and Wainwright. Actually he will be their second best pitcher in WAR.
With those additional prospects the Yankees could get, they could use them plus our own prospects to try trade for Soto and Castillo or at least Castillo and Happ without depleting our farm system.
We could have the following lineup for the playoffs:
Soto (still a long shot) or Happ RF
Carpenter or Donaldson 3B
IKF SS (or Peraza if not traded)
Starters: Cole, Castillo, Cortes, Severino
Bullpen options (the best 9 from): Montgomery, Holmes, Marinaccio, Peralta, Luetge, Abreu, Schmidt, Loaisiga, Sears, Castro, Britton and Chapman.
Let me know in the comments section your thoughts about this, what would you do instead, etc.