One of the pros at my shop tends to use Head Velocity Main and Diadem Synthetic Gut cross hybrid. She goes mid tension for the main, and 1-2 pounds lower for the cross.
What is the benefit of a Velocity main/Diadem cross strung with a 1-2 pound differential?
The theoretical benefit for this pro could be that she gets better control/consistency using this hybrid compared with a full bed of multi. But she still retains most of the softness that comes with the Velocity, since most of us are under the impression that the main strings in a hybrid dominate the feel and performance of the string bed. While some players might get this by using a poly cross with a softer main, others may wish to avoid the poly options completely... myself included (but I string my own frames with full beds of syn. gut).
I think that these hybrids using different soft strings are a mixed bag though. Unless you find a specific combo that really clicks for you, it's apparently a little bit of a gamble. Some pairings get along okay, but other don't - they can be a little grippy against each other instead of sliding over each other with relative ease, which most folks want with a hybrid.
The differential in tension with that pro's setup you mentioned is probably because the syn. gut cross is just a little more stiff than the Velocity mains, so dropping a pound or two gets the overall firmness of the string bed closer to a full bed of Velocity... in theory. Sometimes just trying this or that with various tensions can produce something that seems "right".