I'm trying to replace/modify a series of valves that reduces pressure from 1600 psi to 400 psi. There is currently one PRV along with an isolation valve in series. Both get steam/condesate cut in a matter of months. These valves are cycled approx. twice a day.

Will replacing the isolation valve with another PRV help the situation? This would allow two pressure drops across two valves from 1600 to 400 psi.

Also, does steam cut valves when they are cycled or condesate or both?

What exactly do you mean by cut?  Is the trim damaged/failing?  What are the materials of construction?

How do you plan to control the two valves in series?

Typically, a two phase mixture at the inlet to a control valve is a no-no and should be avoided.  Perhaps you could provide a bit more detail on the application.

I've only seen the valve repaired once, and at that time the valve seat had developed a groove (it is a high pressure control globe valve). Both the seat and plug is 416 SS.

I would use a split range controller to control both valves.

There is approx. 16 ft of vertical 2 inch pipe down from the steam drum to the control valves. The steam drum contains 1600 psi sat steam. These valves are used twice a day for about an hour. While the valves are closed, I assume that the 2 inch vertical pipe begins to fill with condensate. There is no steam trap upstream of the valves.

For a properly design/located sootblowing valve, there shouldn't ever be condensate flowing through the valve.  The steam actually becomes superheated during the pressure reduction.  

It's very likely that the failure is a result of wire drawing/improper valve sizing.  You may be able to just change out the trim (different design/materials) without buying a complete new valve.  Talk to the OEM for guidance on this.  The use of a hard faced overlay will help resist the wire drawing; however, it will still fail eventually.  If the valve is mis-sized, correct that problem first.

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