Is it possible to put two NPCs on the same X,Y position with Plotscripting?

Yes!

set NPC position (who, X, Y)

That will move the NPC "who" (either an NPC reference, or an ID (in which case the first NPC of that ID will be moved)) to the tile (X,Y). It does not matter whether there is already an NPC there, as you could have all 300 NPCs on the same tile, if you wanted!

If you want to position an NPC via pixels, you can use this command:

put NPC (who, X, Y)

"put NPC" has the same usage as "set NPC position", but is measured in pixels rather than tiles.