I am going to build a double door for a barn that slides on channel. The will each be 6' wide and 12' tall. Any thing I need to know?

If the doors are being made with ordinary construction grade 2 X lumber , then the styles & rails ( which are the vertical & horizontal pieces respectively) may be susceptible to warping/twisting over the long height and width.
This is easy to combat if you take , say a two by eight for the rails and styles and rip them into several equal pieces . Then swapping alternate pieces end to end , so that the grain goes in a different direction with each piece ( the way a butcher block table looks on its end grain). Waterproof glue every touching side together , clamp overnight. Ensuring everything is laying flat and straight. It helps to clamp the laminated frame pieces to a straight table in addition to clamping the overall assembly.
Once the styles and rails are joined , you can fill in the door with plywood or tongue and groove 1 X material .
Belt sand smooth.
This will keep the doors nice and straight over their lifetime.

Thanks for the advice, but you lost me at end to end. Do you mean that one swaps the ends. I was going to use one buys for the styles to lighten the doors and make them not as wide. Is this wise or will this increase the chance of warping?

If you take any piece of wood and rip it several times to get equal pieces, you rearrange every piece of the new assembly so that the grain ( when viewed end on ) goes in a opposite or different direction as the adjoining piece.
Flip alternate pieces end to end and rotate each piece as necessary .
Look at a butcher block table end grain , that will give you the idea.

I'd say one byes will not be robust enough . A door that big in 1 X will not last long

Personally I am lazy --I pitch the tracks up at the ends , install counter weights , and have a door that's almost on the ground closed and clears the ground as being opened. Really easy to open to.