Simple answer: you can't.

Not so simple answer: Use the tileset extractor to extract the bitmaps for both tilesets. Then rearrange the tiles as you see fit. Make sure that the mask file fits the new arrangement. Run the result through TilesetPal to get a palette that works. Then re-compile in JCS and build a level with the new tileset. If this terminology is strange to you, you should read through Disguise's Tileset Resource first. The result will probably look worse than expected, as tilesets can only use 256 colours (actually even less), and probably both tilesets combined use more. But it all depends on the tilesets you are merging.