You can use as many tilesheet as you need. The only limitation is that a tilemap can only use one tilesheet at a time (but can be swapped at execution time).
API are as follow:
Texture.setTilesheet(image); // set the tilesheet for this texture only. set image to null to unlink the tilsheet and use the default one. Texture.setGlobalTilesheet(image); // set the default texture globally // Note: the main screen is a Texture accessible with the global $screen variable
TileMap.setTilesheet(image); // swap the tilesheet for this tilemap
Note2: images can be another texture. So it is possible to procedurally generate and use tilesheets.