I suppose, that if the tilesets are from what's shipped with the game, you can combine them, but you still need to give proper credit to both tilesets as if you'd used the tilesets entirely.
If you got the tileset from another place, like posted on the forum or from another level, you will need to ask permission. Credits are also necessary of course.
Best is to always ask for permission, even if the tileset is shipped with the game.