</font><font color="blue" class="small">( While I don't think balancing those blades is very critical, I did just what BB TX said. When I removed my blades and sharpened them a bit on the bench grinder, then I weighed each of them on the same 20# scales I had; ground them to a point that the scales showed they weighed the same. )</font>
If you want to take Bird's method one step further (and correct for the angular momentum concept mentioned in another post):
In addition to making the two blades weigh the same, you need to make sure that each blade balances at the same point... that is, the balance point of each blade should be at the same distance from that blade's bolt hole (rather than trying to balance it at the midpoint as you do with a single, double-ended lawnmower blade). Grind a bit off one end or the other of the blad to move the balance point, then recheck that blade's weight & balance against the other blade.
As Bird said, however, it's probably not that critical. The bearings in a rotary mower are built to take a lot more abuse than in a finish mower (I believe a rotary mower spins more slowly than a belly-mount finish mower as well).
John Mc