I was messing around with Dreadwind and Blackwing, now that I finally have both, and have found a couple of ways to make the combined jet mode more stable.
First fix:
(more stable, but less good-looking)
It turns out, if you pivot Blackwing's shoulders up, (one click short of all the way up) you can get the notches on his arms to match up with the tabs on Dreadwind.
You have to kinda angle Blackwing's arms a bit to get them to connect...
But they DO hold.
This, of course leaves Dreadwind's arms nowhere to go, So I angled them under his wings a bit.
It doesn't look great, but you can add guns to offset it a bit.
I could seriously grab Dreadwind by the jet-nose, and shake them lightly, and nothing moved at all.
Second Fix:
(Marginally less stable than Fix 1, but much better-looking)
It turns out, that part of Fix 1's stability comes from the pressure of Blackwing's shoulders against Dreadwind's feet. So if you swivel his arms around in a different way, it leaves room for Dreadwind's arms to go where they're supposed to.
And it looks much better in super-jet mode than Fix 1. Blackwing's arms just barely peek out from under Dreadwind's wings, and they look fairly natural there, like some sort of wing-mounted weapon or something.
Add guns if you want, but I think it still looks fine without.
It's only marginally less stable than Fix 1. I grabbed Dreadwind, and shook, and they only wiggled a teeny bit.