The problem with the one-legged Romanian deadlift (RDL) is that it hasn't traditionally been trained as seriously as it deserves. It's a tremendous movement that can offer great benefits.
But we also need to perform accessory work for the one-legged RDL. Ask any powerlifter worth their salt and they'll detail the exhaustive work they do to support their main lifts, and the one-legged RDL is no different.
So how do we support the one-legged RDL? What can we do outside of the exercise itself to promote better performance, specifically single-leg stability? Enter the split-stance row. Once you try it, you'll wonder why it isn't already a staple of your training.
Set up in some variation of a split-stance. My preference is the corner stance shown in the videos below. Aim to have your knee over your mid-foot.
Split-Stance Dumbbell/Kettlebell Row
The goal is for 95% or more of your weight to be on the front leg so it doesn't turn into a balancing act. Use just enough rear-foot contact to maintain a stable position.
If you're using a dumbbell or kettlebell, you'll row it on the lateral side of your front leg. If you're using a barbell, you'll center it on your front shin. From there, you just row. Do the opposite leg (and arm if you're using dumbbells or kettlebells) on the next set.
Once you've got the hang of things, you can make it spicy simply by increasing the angle of your front knee.
Split-Stand Barbell Row
Split-stance rows are an effective way to prepare yourself for split-squats, lunges, or, of course, the one-legged RDL, especially if you're a bigger guy and your own body weight is challenging. Four to eight weeks of these will substantially increase your readiness for more unilateral lower-body work. Keep the reps in the moderate range, 6-10 per set.
Once you're performing single-leg exercises like a champ, you can keep doing split-stance rows as an accessory move to build truly formidable single-leg strength. Of course, if you're already primed for one-leg work, start doing one-legged RDLs as your primary movement and do split-stance rows as an accessory movement.