That's a good idea to try them out with one ball first, when I had my two gerbils we started out with one ball which they took turns in. One of them was a much faster runner than the other. After they both tried it out and figured out how to roll around in it, we got a second ball for them.
Here is a photo of my RB gerbils Chocolate and Brownie in their balls.

Your gerbils will need the medium sized balls like your Syrians use.
Their tails take up a lot of room in the ball when they get rolling.