Since these two are written as potential crossover adventures, has anyone run them as such? How did it go?
No, I've run only certain adventures from Tales from the Infinite Staircase.
Linking these two modules should work fine - the biggest question is probably if the PCs are primes or planars.
If they are planars I'd start with Tales from the Infinite Staircase, if they are primes I'd start with For Duty and Deity (in which case you'll need a somewhat spectacular explanation for the Iron Shadow as it's hard to impress people who have just saved a goddess from the clutches of a demon prince).
I never understood this idea. Both adventures on their own are excellent, but they don't compliment each other in any way, other than that they're set in the same location.
In the end they're less like puzzle pieces or lego, and more like bricks. Sure you can stick them together, but you'll have to do it yourself, and in the end it's still just two bricks.
Staircase is modular enough, but Duty is pretty much one big chunk. So you can arrange the modules from Stair around Duty, but you could do that with every old adventure.
You see, Duty puts players on a bit of a clock, and the Staircase isn't timeless. There's some room for fumbling about and finding the right door, but not enough for whole planar safari's.
For Duty and Deity's hook is a desperate plea for immediate help. (Funnily enough this urgency is then immediately frustrated by having the players wait, while the NPCs deliberate, but I digress.) Eventually all the clues are uncovered, and then the PCs are ushered onto the staircase. Their way there is led by an NPC who holds all the keys to all the doors, willing to aid them for this mission; the players are probably going to have a hard time getting there on their own leisure.
I'm not too clear on the details of the stair-trip itself, but iirc it's a four-day hustle, with one iconic overnight stay, and a bunch of random encounters. Also iirc, not much effort is put into letting the players find the right way, it's either through an NPC guide, or through wibbly-wobbly-spatial-weirdness-boom-you're-there-DM-magic.
Once through the door, there's little incentive to go back to the staircase before the mission is completed, because the clues begin piling on that time is of the essence.
With the mission complete, I guess there is some time to wile away on exploration of the staircase, although by that time the party will have a very impatient NPC in tow. Once they deliver that one home, I don't think the party even return to the staircase, but they are simply zapped back home.
Duty has some staircase elements that you can use in Stair, and Stair has plenty of material that you can drag into the section of Duty that's set on the staircase, but I'm not sure that's enough to warrant a true crossover label.
Sorry for the rant. It's not that I hate either adventure, it's just that I too believed in the idea of merging them, and ended up a little disillusioned.
I agree with Glim. It seems possible to play one after the other, if you can find a convincing reason for the PCs to do so, but I don't see how you could actually interweave them with each other.
The best solution would probably be to have a planar party (including a PC with ties to Toril or Waukeen) play through Tales from the Infinite Staircase. After this the PCs might beconsidered as "Staircase experts" and are thus chosen to rescue Waukeen in For Duty and Deity.