Use two hammers, one as a backup and gently tap on the conection flats. Put one wrench in position and lay it all on some hard ground. Place the other wrench so that you have a scissors effect with the wrenches. Place one boot on the top wrench and apply pressure. Be careful your foot doesn't slip as it pains some when the rench hits your shin.

24 in pipe wrenches would also help.

Egon