Have you got any kind of wind-down routine in place for when you want them to sleep? If not, I would definitely consider introducing one. 9 weeks is a good time to introduce it, but you may need to be prepared for a battle - probably best tackled while you are still at home rather than when your wife has to do it single-handed.
The routine typically suggested is bath, feed, story/song, cuddle, bed. The bath you may not want to do every night (I only bath ours twice a week) but the rest of it would be every night, as close to the same time every night as you can manage. All to take place in a calm environment, so dim lights, nursery rhymes or soft music playing, quiet voices, no toys except soft ones. And do try to make sure they are sleepy but not asleep when you get them into bed - if that means you have to skip the story, fine. Then a kiss, a few quiet words (eg night night, sleep tight) and tell them that you'll just be downstairs.
We introduced this at about 6 weeks' adjusted age (which was 14 weeks chronologically) and took the pain for only a couple of days before they got the message and dropped off when laid down. They did scream relentlessly the first couple of times we tried it, it was horrible and I doubted it would ever work. To be fair, I had not got into the habit of cuddling them to sleep, but they had been in the downstairs rooms with us and got used to more noise/light around them so that was still a habit that had to be broken.
Now - touch wood - they go down really quite easily every night, and for their daytime naps. They are used to their room, don't even need it to be pitch black in there as some books suggest, and intervention from us is very minimal. The key, I think, is to make it all very predictable i.e. reassuring but also dull so they're not incentivised to stay awake.
Another thing that may help is to lie their heads on a muslin, which we only introduced to catch possets but my DS (the more agitated of the two when he wakes up) has begun to use it as a comforter and it seems to help him self-soothe if he does startle for any reason.
And my final recommendation is a CD of womb sounds called Babysooth, which my Mum got me. I think it suggests you should introduce it before your baby hits 12 weeks because it will be more successful then, and I have to say it works like a charm. If they wake up early and I need them to go back to sleep but they are resisting, I put this on and even now (5 months) it is like baby opium to them. You can get it from Amazon for a tenner. Well worth it.
Whatever you try, don't assume it hasn't been a success if it doesn't work immediately. Give it a go for several nights - they need to get used to something and not have things switched too quickly.
Good luck, and I hope your stress is improving.