I had this exact problem with our John Deere just before Christmas. It took a long time to get them freed but in the end I did it. This is how I did it:
1. Squirted plenty of penetrating oil on both blade bolts and left over night.
2. Heated the bolts up for 5 mins.
3. Turn the bolts with a ratchet in the opposite direction just slightly to get it to move a bit.
4. Turned the bolt the opposite direction in the way to unscrew it and added the longest extension onto the ratchet. (If I had one I would have used a breaker bar).
5. This did it for me but if all else fails I would repeat the process.
Hope that helps.