VEVOR 8FT Sliding Barn Door Hardware Kit, 330LBS Heavy Duty Barn Door Track Kit for Double Doors, Fit 3.7-4.3FT Total Wide and 1.3"-1.8" Thick 2 Door Panel, with Smooth & Silent Pulley (I Shape)

It is recommended that the track needs to be twice as long.

Yes, but not as supplied. This kit is designed for two sliding doors with a combined door width of 3.7–4.3 ft. It includes four hangers (two per door) and hardware for a double-door installation. If you want to install one 4 ft door, it's better to choose VEVOR's 8 ft single-door hardware kit, which includes the correct hardware for one door up to about 4.3 ft wide If this answer does not resolve your issue, please contact us again for assistance.

They just have different hook shapes, but the functions are the same：https://send.now/a/ZCN

Question should be answered by the manufacturer. But 16inches is standard.

The holes are 16 inches apart, yes.

Yes. I woud use this for exterior. The metal is solid and the moving parts are made of solid plastic material. I have a similar system that as been outside in all weather conditions for 7 years now with no loss in function. Even the metal parts are only just showing some minor rust.

I have not used this hardware outdoors. It has a very good coat of paint which I believe is powder coated, so it should do well in the elements but you can expect that the bearings will eventually get corrosion in them. A good maintenance plan for lubrication from time to time would be a good idea.