First of all, the main reason for using a hybrid string is because you need a combination of benefits that one string alone can not provide. An example of this is someone who wants the durability of a poly string but needs a softer feel than poly alone will have. By pairing the poly with a softer string, he can get some of the benefits of both strings and reducing the negatives in both.
As for what to use, a good way of thinking of it is apples and oranges. Most often the strings will be fairly different. Some players will use two poly strings as a hybrid, but that is like using apples and apples. The majority of the hybrids we string are a stiff, durable poly string paired with a softer, multifilament or synthetic gut (in feel and performance, apples and oranges). Using two very similar strings, while not out of the question, will not give you the benefits of an "apples and oranges" hybrid like improved feel and added performance.
Stop in and we'd be glad to help you determine if a hybrid string job would be best for you and what combinations will have you playing better tennis.
