I doubt if that crankset can be welded back correctly as how will you do a full penetration weld on it. You'll most likely end up with a weld near the surface and still a crack inside the arm which is not 100% good. You almost have to gouge out material close to the full depth of the crack and then fill it with weld, then grind/polish it back to spec, then you still have to grind out, clean out and re-tap the extractor threads for both. Lots of (expensive/labor intensive) work and who knows if you might end up overheating the part and end up with something weaker.
I think the crankset's toast, time to look for another one....
BTW, did you check if by any chance the cranks were torqued down on the spindle too far/much which might have contributed to the cracking??

Chombi