The antennas need at least 3' of space between one another. Usually UHF at the top and VHF below yields the best results.
My most recent tripod install was a 5' (nominal) Rohn unit. I took the time to add structural blocking on the underside of the roof deck using screws and glue to transfer the load to the rafters and to have plenty of material to sink lag bolts into as I secured each foot to the roof deck. This has given me a solid base for the 10.5' chain-link fence top rail (1.375" OD) that I chose for the mast. I've got a total three antennas and other accessories on the mast and have been through several storms with no guy wire.
However, adding guy wire is certainly a reasonable option. Here is an example of an off the shelf clamp that is intended for such an application:
http://www.3starinc.com/adjustable_3...enna_mast.html
Common eyed lag bolts into the roof deck and rafter or blocking would be plenty of anchorage.
Of course, you'll want to seal with an appropriate roofing cement/sealant and check/service it regularly to avoid leaks.
I also installed an antenna for my church. I fabricated a non penetrating base and used 1" IMC X 10' electrical conduit for the mast. I also fabricated a wall bracket to support the mast 3' to 4' above the base. (But this is on a flat roof that has a low 'penthouse' like section next to the antenna installation. No guy wire, and the system has made it through this winter including a couple of tree breaking storms.
