First, I assume that the cypress is 1x6 and not 2x6, correct?
Second, I hope that you are not using 2x4 for the rafters. You need to use at least 2x6 and probably larger depending on the span and local codes.
Technically you can install the metal roofing as you suggest, but it may not be wise, may not be allowed by code, and may not be the the method suggested by the manufacturer. In other words, don't do it unless it for a barn or shed! I suggest that you install plywood panels at least to improve the structural rigidity and integrity of your roof. Then install tarpaper (if suggested by the manufacturer) and the roofing.
Contact the roofing manufacturer and they will tell you what you should do. Talk to your building and permit agency and they will tell you what you HAVE to do.
Also consider whether you should have ventilation under the metal roof and a ridge vent -- discuss this with the manufacturer.