fencegate-postgates
I have just built a 6 foot fence all the way around my property. To finish it offI need to build a driveway gate. The gate will be 15 feet wide and have two swing gate sections. I have used 4x4x8 foot posts buried approximately 26" and cemented.
I was considering bolting two posts together and burying them at the same depth, but I am concerned that it still may not be enough support. The gates will have wheels on the bottom also.
Rule of thumb is that 1/3 of the post should be in the ground. A 6' fence should be sunk into the ground 3', so you'd need 9' posts.
As Shirlock states, even that won't likely support an 8' wide gate. Two 4' gates would definitely be a better option.
Perhaps the easiest solution, however, is to not even make a gate. Instead, build your fence and then make this an 8' panel that is attached with temporary fasteners...such as bolts. On the rare occasions that you need access, unbolt the fence panel and remove it completely--thereby negating the need for the two posts to cary the load of a cantilevered gate.
On the plus side, it's less gate hardware you need to buy and deal with.
Eight feet is a common distance between posts, due to it being a standard lumber length. You will have a stiffer, longer lasting fence at 6-7 ft. An integer number of pickets with your desired spacing is a good way to decide from a design standpoint.
I prefer to sink posts to at least frost level (42" in my area). The 1/3 advice is good if frost is not an issue.
I also prefer crushed gravel (21AA or paver base) to place wooden posts (versus concrete or pea gravel). 3-4" in the bottom of the hole (beneath post), continuing up the sides of the posts to just below the surface. Give the post a push after first 5" and tamp along all 4 sides as you fill. This technique will greatly extend post life and minimize rot.
I've used the gate brackets that you build into the gate which have hinges built in. They will stabilize a 42" gate (each side). Extra long screws on the top hinge are helpful. Stainless steel cable run diagonally (with adjustment turnbuckles) run from the top of the hinge to bottom of the gate (at the opposite corner) will make it sag proof.
Disclaimer 1 user of product, no financial connection: 
Disclaimer 2: Haven't used it but sounds interesting, YMMV (Near) Instant fence post backfill
Best Answer
The rule of thumb I know that applies here tells me to use at least 6"x6" posts for wide gates concreted into a 3ft deep hole. By preference I'd go for a single larger post and go deeper than you suggest. You might get away with firmly bolting the two posts together, but I would be wary about compromising on the depth...