Doors and Windows - framing double windows

I'm installing two new vinyl 4/0 x 4/0 slider windows in a small workshop I am building and don't have to have a permit so I need some advice on something. I want to install these side by side and build a single sill on the inside. Should I build a single continuous header and will two 2 x 4 trimmers be okay between the windows to support that header?.

My other question regards flashing for doors and windows. I am nailing all of these to OSB sheathing and then siding the structure in T1-11. Is caulking adequate to keep water out or should I have flashing around the doors and windows? Does anyone have any recommendation for these types of products?

Thanks in advance.

What you propose is ok ( 8' is the max opening for two trimmers ON EACH SIDE....) but I would also add one 2x4 in the center. Vinyl is a nice material, but when you get up in that size window and above, it needs a little help. One 2x4 in the center will stiffen it up nicely in addition to giving you something to secure the window centers to something other than another pc of vinyl.

As to flashing.....depends on how the windown is located. If you're back under a good roof overhang and not on the windward side of the building, caulk will do.

If you're on a gable end where rain can hit above easily and run down, I would make a "Z" ( picture it turned on the side ) type flashing for the head of the window installed up the OSB and out over the top of the window before installing the 1-11.

They also make some nice rubber membrane stuff for running around the openings now.....I've never used it, but it looks like pretty good stuff......