[Al Skierkiewicz]

Gary,
The proportioning valves will lower the delivered air so that the return force is less but the piston still receives full 60 psi to keep it in place. The regulator may act slower than the valve approach. You would have to test the response of each one. The only advantage of the secondary regulator is slightly less air volume used.
However, this might be of interest...

<R76> “Working” air pressure on the ROBOT must be no greater than 60psi. All working air must be provided through one primary Norgen adjustable pressure regulator.

A Q&A check would give you a better answer. We have allowed more than one in the past. Specifically in the 2008 rules there were no limits on the number of regulators. 2009 had the same rules as this year.