Have you tried almost herding them together.
Unfortunately this would mean moving them around using your hands/arms to “herd” them.
If Dustin was upstairs for example, I would urge him to make his own way down so they can be reunited, then herd them both towards the bottom of the ramp. Generally speaking they will be a bit nervous and go together, and when he watches Dustin get a bit scared and go up the ramp, 9/10 times he will follow along.
Same goes with the other way round, once they have used it a couple times, they eventually get the hang of it.
I am not 100% sure what the proper method is for this, but I do know baby guinea pigs take things in and are constantly learning.
In a group or in any case of a older pig and baby relationship you will see a lot of mirroring and teaching from a baby, taking in the world and learning what it’s all about through an older counterpart, and this is quite important as you can guess. So as this is taken away, everything becomes a lot more nerve-wracking, I imagine almost like not being given a directions to a important work dinner
Your babies (I am not sure if they are babies) will be looking for company right now, which is why I find sort of forcing them (gently) to take this step a needed/good one.
Journey through a Lifetime: The Ages of Guinea Pigs
I apologise as I am continually bringing it up, but is there anything on your ramp? Or anything underneath the ramp to make it less steep.
Ramps in Cages