How about a big ol' 2 man style chain saw bar and blade. Check out Huskavarna (sp) or Stihl web sites.. Think they still make a 4-5 foot 2 man chainsaw?  Anyway,, get a hydraulic motor adapt to the chain drive gear, build a frame with a rolling "sled" run the hydraulic motor off a tractor remote, set a bale on the sled push on the sled, rolls on it's track, (Bar and chain is mounted vertically in center of sled and frame) slices through center of bale, on "outfeed" end bale become's 2 bales.

If they don't make the "big men saws" then.. How about 2-3 of the longest bar's available,, cut at proper place on bars,, then make a single long bar. Downside would be welding back together would require retempering the bar.

considered 2 offset bars but,, it would be too hard to hold the pressure of pushing the bale through on the sled.

Plan "b":

Know of any sawmills?  thinking 2 offset 3 foot circular saw blades running off either a chain gear setup or hydraulic motor.  Know saw mills still use BIG band saws to slab the trees on a rolling sled but, those big band saw blades are expensive.

With big bales seems like having the "cutter" stationary and the bale passing through the cutter on some sort of rolling sled would be best.