I have two gerbils also but mine are both females, their names are Chocolate and Brownie.
I have had them for 6 months now so they are just a bit younger than your gerbils.
What I found works really well is to have them run in exercise balls outside the cage.
Just give Al more time in the ball than Sam and he should slim down as he runs around more.
My gerbils never liked their exercise wheel in the cage either so this works really well for my gerbils.
Here is a photo of them both rolling around in their balls.

I found one of the balls at Petsmart and the other one at Petco.
Do you have either of those stores near where you live?

I would love to see photos of your gerbils if you have them and good luck in getting Al to slim down.