First of all, I am not an architect or an engineer. However, I will share with you how I would go about tackling your job.

You don't say what depth joist you have there in your attic. Nevertheless, what I would do is get two joist sections up there, butt them together at the ends making sure that the other ends are supported by the load bearing walls. I would then either join the two sections together with an eight foot joist or an eight foot 1/4" thick steel flitch plate of the appropriate depth to act as a mending plate for the two sections. If using wood, glue and nail the board to the sections. If using the 1/4" steel flitch, lag bolt it to the sections in a bolt pattern punched out by the steel company you hire to make the thing for you. It is difficult to describe, but what you wind up with is holes for the lags every 10" or so in a regular "up and down" pattern across the flitch.

2x6 are the joists in the attic. What you say makes sense, if I truly can't fit a single 14' 2x6 up there (maybe I'll give it a shot and see it it'll angle up in there somehow), then I'll probably go with the wood-joining method...

You are completely misunderstanding the instructions.

When Step 2 says: "The double joist sections shown in Figure 3 must be long enough to be supported by a load bearing wall at both ends."....All it is saying is that the double joists must bear over each hallway wall...

This cannot be more than 4 feet wide in most cases.

You should already have a single joist at the end of the rough opening once you cut the joists.

All you need do is add to each another joist section that will go from the top of one hallway wall plate and bear on the other hallway wall plate by at least 1 1/2".

No 14' joists.

Just joists the same depth as your existing joists that can span from hallway wall to hallway wall....

Manhattan - If you are correct, then wonderful....but it does say that the double joists must be long enough to be supported by a LOAD BEARING WALL at both ends. You are correct that 4' wide would cover both hallway walls, but only ONE of them is load bearing through the garage beams one floor beneath it. The other is just an interior wall with no support in the garage. So wouldn't that be considered non-load bearing?

Wouldn't that mean that in an absolutely WORST case scenario, the garage ceiling underneath that non-load bearing wall would potentially sag over time?

Please correct me if I'm wrong, thanks for the help.

I agree with Manhattan that you're reading too deep into the instructions. Unless you planning on putting bricks up in the attic, a header on each side for continuity across your new opening is fine.

There is no need to try to run a 14 ft board to an exterior wall.

FWIW: ask any reputable contractor how he installs these things and how many problems he's had. You might even just have a contractor do it. Simple 1/2 day or less job.

That "interior" partition wall has enough strength to span that distanse without sagging, unless its made out of cardboard.

My 0.02.

Are they really floor joists or the main stringers of roof trusses?

If they are roof truss stringers you not only can't cut them, you can't bore holes or notch holes in them to run a pipe through without the expressed written instructions of the truss manufacturer.

I felt like I may be reading too deep into the instructions...my neighbor didn't double up or anything and has had heavier wooden attic stairs in his house (same structure as mine) for 15-20 years with no garage sagging, so I guess there's my proof in the pudding.

But, no roof trusses, just joists, rafters, and collar beams.

The interior partition should be plenty fine (heh, cardboard...), and I won't be storing very heavy stuff up there. Just the basics like empty suitcases, seasonal clothing, old books & toys, etc. And I'll try & put down the plywood on the other half of the attic or on only the uncut joists, would that help?

The 4 cut joists only span 1/2 the attic anyway, there are joists that also run from the other half of the attic, and then both halves overlap (side-by-side) over the single load bearing wall, they are nailed together where they overlap, who knows if this is even correct, but it's the hand I was dealt...

So unless I feel adventurous and want to drive home from home depot with 6' of wood sticking out my sunroof, I'll probably just span the 2 interior walls with a 4' 2x6 and call it a day....

You won't be able to splice joists.

You originally said you were cutting 2 joists, then you referred to 4 cut joists. How large is the opening?

For 2 cut attic/ceiling joists (4-0 span):
If you use a single 2x6 header (with joist hangers) to support the 2 cut joists, the light attic storage design load from each of these shortened joists (about 170 lbs each) will be transferred to each end of the header where it will be partially supported by the "non-bearing" wall which will transfer the loads to the floor joists below. The joists below will be strong enough to support two 140 lb point loads 4-0 apart if they are 2x10's @ 16" o.c. I would also be concerned about cracking plaster if the full-length ceiling joists don't touch the top plate of the "non-bearing" wall so check that detail.

For 4 cut attic/ceiling joists (6-8 span) you will need to hire a structural engineer.

It's also possible to support each joist from a rafter if the rafters are large enough and/or well enough braced which would require an engineer to determine.