Ha, fun with upright pianos. If you only have two SM57's, I'd recommend that you open the top and place two 57's on booms pointing into the piano angled towards the soundboard. Place one over the treble and one over the bass strings. A dirty, relatively cheap, and easy way to get a good sounding upright piano is to tape a boundary mic (like a PCC-160 or my favorite SM91) upside down into the top of the piano and then close the lid carefully avoiding crushing the cable. That's what I would normally do.