Its discussed in detail in this thread:
Strain ellipse mechanics adjacent to clamps
If I remember right he used then a "cock coffin" whre he had several clamps glued together giving strain to many points over the penis.
Now to start this the easy way I would for the most effect place the second clamp on varying positions.
If you just want to try it on one spot, like midshaft, it would show you ifits really efficient.
Of course you need to be very cautios as the second clamp can put enormous pressure.
I use two clamps at the base and can vary my pressure like I want(its more complicated with only one clamp). The best is that the clamps are always now on the very base without moving up. I switch the clamps and their positions with each workout to get an even strain.
I use my hands to simulate another clamp over the shaft with little squezzes.