I agree with everything mshilarious said, and would like to add the following variants...
If you really need to use the Shuresm take the ball off the 58 as ms suggested and try miking them as a stereo pair. Mic stand them into the piano a half a foot or so below the hammers, facing down towards the strings and angled slightly up towards the hammers. Alnge the mics left and right a bit with the 57 facing towads the lower octaves and the 58 towards the higher. Move the mics closer to the hammers for a more rock n roll/honky tonk sound, move the further away for a mellower jazz or classical sound. Adjust to taste.
And, frankly, be prepared to be dissatisfied with the sound you're getting. If you're looking for a raw "Jerry Lee Lewis", you might get something servicable from the Shures, but the further you want to get from that sound, the more dissatisfying the sound of the Shures will be.
Alternately, you might just want to mic mono with the AT. Alternate locations for that could be at the foot of the paino, just outside the lid facing back in towards the piano and player. This position can take some expirimentation as a couple of inches either way can make all the difference. If your room is large enough and pleasant enough sounding, you might want to back off the AT totally outside the nearfiled of the piano. This would mean placing the AT at least 7 feet away from the piano on the side facing the lid opening.
Depending on the sound you're lookimg for and your environmental conditions, though, be prepared for the very real possibility that you might need to buy, rent, or borrow two or three of condensors (LDC or SDC), piezos or a mix thereof. I have had very good results with
a Rode NT4 dual SDC inside the piano in the placement described for the Shures above, with an LDC distance miked outside the nearfield for optional fill.
HTH,
G.