Try doing RDLs with your Scapulas retracted (you shoulders blades pinched). When i started doing this, i felt nothing in my back period when doing romanians. You also might want to try stretching your hams a bit more. Some back discomfort is caused from tight hamstrings.
If you're training them twice per week, you could focus on mostly quads in one session, and hamstrings in the other.
A example could look like this:
Quads Focus + Hamstring flexion
A. Front Squats...ramp to 3-5 RM **Could also throw in some sets of bodyweight vertical jumps between sets, to activate explosiveness if needed....wouldn't need to do them between every set, just when you feel you need to "wake up" before a big set...usually early in the ramp
B. Back Squats...ramp to 3-5 RM..starting at weight you ended with on Front Squats **Box jumps (choose a height that's fairly challenging)
C. Hack Squat Leg Press....Could go with a higher rep range....8-12....I prefer higher reps on leg presses, since you can move quite a bit of weight, don't have to focus as much on stability, and it's a great mental challenge to keep pushing out the reps
D1 Hamstring curl movement of choice....lying, seated, unilateral...I usually keep hamstring flexion exercises under 8 reps max....if using a decent amount of weight and being as explosive as possible, hamstrings can only be truly explosive for just a few reps
D2 Calves of choice....if you include jumping exercises during the front and back squats, you don't need quite as much volume, if not jumps, hammer them
A few days later could be: Hamstring Focus (mostly hip extension)
A.Sumo Deads.....Ramp up to 3-5 RM **I throw in some standing broad jumps (2-3 explosive jumps) between sets for activation and to keep me in the groove while "resting"---not necessary, but will help if you don't overdo the jumps
B. BB RDL's....8-10 reps....your posterior chain will be plenty activated from the Sumo's, and shouldn't be fried if you regulated the deads properly and used sound technique
C. High Step ups (working leg knee should start above the hip....this will have the hamstrings and glutes start in a stretched position which will involve them more each rep).....I like doing 5-8 reps per leg for these **Could also do long stride lunges, bulgarian split squats with a large stride distance from bench behind you, or reverse lunges from a deficit...IF flexibility and coordination allow
D1 Quad Extensions.....good time to use a higher rep range (10-15)....quads respond well to higher reps, and this being a single joint movement (only one for quads), would fit in nicely here. **Quad extensions are great for pre-exhaust, but you can't really push your quads to their limit, due to the squat work done after. This is a good time to get in some additional focused quad work, which will add extra volume, and it's another great opportunity to push yourself mentally, as these burn
D2 (Could do some higher rep range (10-15) hamstring curls, using a movement you didn't use on Leg day 1 OR if you're hamstrings are fried from the previous exercises, you could do a Calf movement here (One you didn't do on leg day 1)
There are a million ways to split up your training, but the basic principles should be the same
This is just an example, and you should structure your training according to how your responding. I'm not sure how long you've been training, how in tune with your body you are, if you're regulating workouts properly, getting enough rest, eating well, etc, but the above example would provide a good structural plan that incorporates a variety of rep ranges, sound exercise selection, with minimal overlap. There will always be some overlap when training legs, but by splitting up the focus into a couple of different days, it will ensure you're getting enough direct work for your legs.