Sounds nice! Ideally I'd like to have something like that in EPKL but I haven't made it (yet).

Yes, combo is a very overloaded word. I'd call it a chord! Normal typing is sequenced, while this is a two-key chord – similar to how we chord when using modifiers but without a designated modifier key.

And yes, 'you' is a much maligned word on Colemak. It's a lot worse for newcomers from QWERTY though, as many typists on that layout underuse their right hand pinky. After a while of using an optimized layout – the good ones all have higher pinky usage – people's pinkies get stronger and the problem is lessened. But it's still a bad thing.

I actually noted while typing 'layout' just now that I often mistype that word as 'laoyut'! Most of my typing errors are shufflings like that. I think this nicely illustrates that the OY/YO bigram isn't easy even after many years with Colemak. There's not much for it I believe, it's two bad fingers in a row.

Short of an ingenious solution like yours, I'd suggest compiling a training list of words containing 'ou uo oy yo uy yu' using for instance GFruit's word filter:
https://gfruit.github.io/typing/words-filter.html

Here's such a list I just made for training as a Monkeytype Custom text:

about alloy aloud amount anyone around beyond bought bounce bound bounds boy boys buy buyer buyers buying buys canyon cloud clouds cloudy colour couch cough could count counts county couple coupon course court courts cousin cowboy deploy double doubt duo employ enjoy enjoys enough famous favour flour fought foul found four fourth ground group groups guy guys honour hour hourly hours house houses humour joy labour layout liquor loud lounge loyal mayor mount mounts mouse mouth noun ought ounce ounces our ours out outer outfit outlet output pouch pound pounds pour proud quote quoted quotes rouge rough round rounds route router routes royal scout scouts should shout sought soul souls sound sounds soup sour source south soy spouse though touch tough tour tours toy toys trout voyage would wound wounds yoga you young your yours youth

The most common usage I've seen for chords like that so far has been special symbols like accented letters. But one thing I'd like to chord would be Ctrl+Back! Being able to delete the previous word with a 2–3 key homerow chord would be very nice.

If I were to do that I'd probably use TO since Ctrl+Back on Extend is Ext+T+O. But then again, it's best to use one of the rarest bigrams (in both directions) on the home row to minimize misfires for something as destructive as deleting a word, and I don't know what that would be. Maybe not homerow but just an easily accessible pair of keys? Or a three-key chord for safety?