I have been a regular disciple of the 2x20 @ 90-95% FTP church for some time. I have gotten to thinking about mixing in some over/under sessions into the mix. Something like 1 minute @ 95% FTP/1 minute@105% FTP for the 20 minute periods. End goal is improve recovery when in and out of the red zone on spirited group rides or just when I want to try to make my riding buddy suffer a bit. I am not racing any tris this year, just focused on having fun on my bike. Who else has experimented here? I am curious if that window is enough to elicit a physiological response other than slightly more accumulated stress and slower recovery than from a typical 2x20. Maybe make it 110%/90% as the over/under? Or change length of over/under periods to maybe 5 min under/5 min over? Suggestions and feedback welcome.
What you describe will work.
I did several different over/under workouts on Trainer Road this winter. They are extremely effective. There are lots of variations. 95-105% for 1 min each is a good one. 85 for 1 min/120% for 30 sec is another good one too.
They are harder than you’d think since you can build up a lot of fatigue as the intervals progress, especially if you go higher on the overs so you are much better off breaking up your 40 minutes of work (2x20) into 8-10 minute intervals with decent rest in between.
Also, while the “over” parts are hard, it truly sucks to come off the “over” and only go back down to 95%. After a while the start of the “unders” start to feels like you are barely grabbing onto a cliff face with your finger nails staring at an abyss. When you crack, it comes when you can’t stop at 95%, not when you can’t get up to 105% 
I love over/unders.
The ones I do are 3x15’ (5’). The 15’ intervals are: 3x 3’ @ 90-95%, 2’ @ 105%. Time goes quickly and you get lots of hard work in.
My favorite workout is ten times 15 seconds at 200% and then 2:45 at 80%. Rest 10 minutes at 60% and repeat. Holding the 80% is the hard part. It forces you to recover while still putting an effort into the pedals.
This is one of the hardest ones I’ve done on trainerroad… VERY effective
https://www.trainerroad.com/cycling/workouts/1817-Mary-Austin-
For a shorter trainerroad workout, look up the Bear Creek 45. It is a 45 minute 3x9 over/under workout. You do 3 sets of 3 over/under intervals at 95% and 105% of FTP, 1 min under/2 mins over x3, 3 min rest between sets. As somebody said above, it isn’t the over that gets you, it is trying to come back and hold the under after a few intervals of the over. It can be a deceptively difficult workout.
Out of curiosity, is there any evidence that over-unders are more effective than normal steady state threshold work?
Effective at what? Raising overall FTP? Improving ability to handle surges in pace? Something else? Whether they are more effective depends on what you are using them for. I think they help me handle shorter term slightly harder efforts like surges in pace to pass or shallow shorter hill climbs. That is why I like the shorter duration 3x9 sets instead of longer 5-10 minute ones. But I haven’t seen any scientific proof that it is any better for that than normal steady state work.
If you are a bike racer or ride in groups they are highly effective because, well, you actually have to vary your pace a lot to be successful in those settings. Frankly, any bike race, whether it is a Cat 5 crit or a TdF stage is often just one giant set of over unders. The guys who get dropped are the guys who can do one less repeat than the other guys . . . .
For other folks, particularly those training on a trainer, intervals generally are a much more productive way to spend time. Sure, you can just crank away at a steady state for hours but why do that when you can get your work in in 60-90 minutes?