You currently have ONE rubber flex disc at that end of the driveshaft, as shown in your picture. The second one bolts on right where the first one is, in between the transmission coupler and the driveshaft. This is what two looks like (without the driveshaft installed in the tractor):
Notice all the extra length on the bolts, and that the flex coupling is stretched? That's why.
I have an old JD 214 that I'm converting to electric. Bought it with no motor. The seller said one rear axle was leaking. So ten years ago he wrapped the joint with a piece of kerosene lantern wick and tied a piece of bailing wire around it. Said he hasn't lost a drop of oil in ten years!
No, but they will last longer if there are two of them. The driveshaft needs to have the holes drilled in the correct place to have two. IIRC, there was a serial number break for this.
Replacing worn engine mounts with new OEM, doing the cradle mod as described on this forum, and making sure the couplers and ball bushings in the driveshaft are in good shape will reduce the vibration.
Considering the cost of the consequences if the flex couplings fail, particularly the rear, this is an area where I'd not want to cut corners unless you are certain the aftermarket part has the same capability as the original part.