Why not take a cutting?

You could probably carefully excavate one side of the plant until you can see if the two shoots have separate root systems and then shear off one of the shoots along with part of the roots and tubers. If you planted a single tuber then this will likely not work because the bases of the sprouts would be very close together and difficult to safely separate, but if you planted a clump of tubers then you would have a better chance of dividing it into two pieces.

That means I would have to sacrifice a plant because two shoots from one tuber gives you a shorter, less vigorous plant.
I don't know how to take a cutting? Will it grow if I do that?

It's pretty late in the season to be taking cuttings with the intention of getting fully blooming plants this year. If they were my plants, I'd leave them alone, and plant clearance annuals from all the sales going on right now in the empty spaces you have.

In trying to gain more plants, you might very well destroy healthy growing and soon-to-be blooming plants.

Cheers!
CC