fencepost
I'm building a privacy fence. How would a professional handle a 135 degree corner?
The complication I think I'll have is attaching the stringers to the front facing side of the posts. On one side the stringers will be flat against the post and on the other side there'll be a 45 degree gap. How do professionals handle this?
A solution I saw somewhere was to use two posts for the corner but I'm not sure if thats the best solution.
Heres a sketch of what I'm looking at:
Don't attach the fence to the house. Aside from putting holes in your siding (not a good thing), your fence and your house will most likely move differentially. The attachment could result in a tear of the siding, which would be a bad thing.
Place the fence post close to the house, then run the fencing top and bottom supports up to within an inch or so of the siding (do leave a little gap, for that differential movement). Place the slats on the supports, allowing the slats to get just a bit closer to the siding.
You could even install a flexible membrane, similar to a product used to seal the bottom of a garage door at the floor, between the fencing and siding.
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
I don't see any need for a second post. Here's how I would do it. Incidentally, buy some 3" deck screws for this if you don't have any. You will WANT to use screws here, even if you've been nailing
Congratulations! You've just made a 135 degree turn
Now, if you're scared about the 2x4 splitting (or you got the screw too close to the edge), what you can do is take a spare piece of 2x4, cut some pieces (same number as stringers) the same size as the 4x4 (about 3.5") and bevel it 45 degrees the other way (left hand cut) before you start step #4. Screw these onto the 4x4 and then you can sink 2 additional screws perpendicular to the board you're placing, though these additional pieces and into the 4x4 itself. This will guarantee you a strong corner, but it can be overkill. I'll leave it to you if you want to do this extra step.