OK, so here are my quick thoughts on whether the squat drives the deadlift or vice versa.
The overall answer, as always, is “it depends on a lot of things” (proficiency with each lift, frequency with which you train each lift, how much assistance work you do, etc).
My anecdotal experience: I spent a couple of years doing mostly high-frequency, low-volume deadlift workouts. I actually got all the way to a 500-pound deadlift without doing any squatting, or really much of anything but deadlifts (just a couple medium-to-heavy singles, performed 5-6 days per week) and some kettlebell stuff.
At some point, I decided to start doing a few squats. The first day I squatted, I worked up to a single at 295, just on some carryover from the deadlift and whatever muscle memory remained from squatting 7-15 years prior as a high school & college football player.
I carried on doing mostly deadlifts with some squats sprinkled in for awhile (probably about 4-5 deadlift days and 1-2 squatting days per week). I think the addition of the squats actually got my deadlift moving upwards again, and I hit 600 pounds later that summer. I kept at it mostly doing that for awhile, and hit a 625 deadlift early this year (February). I also eventually got my squat up to 420 pounds, though it wasn’t as much a priority as my deadlift.
This summer, in July, I decided to flip things around & see what would happen. I started squatting (mostly back squats, some front squats) 5-6 days per week with just a single deadlift day.
I’ll be honest, I’m not all that happy with how it’s gone. I’m sore more frequently, my knees and ankles feel a bit more beat up, and my posterior chain has lost more than my legs seem to have picked up. I have moved my squat up a bit (370 front squat & 445 back squat) but I’ve lost a lot on my deadlift (e.g. now 495 feels really hard, and that used to be a medium working weight; I also can just tell as soon as I start a pull that I don’t have the same posterior chain strength & tightness that I did before).
So as of last night, I decided to go back to the old ways: I’ll start pulling 5 days/week (Mon-Fri) and then squatting on the weekends.
What does this mean for whether the squat drives the deadlift, the deadlift drives the squat, both, or neither?
Well, I certainly don’t think the squat alone drives the deadlift. I’ve spent three full months doing pile of squats, still pulling once a week, and it feels like I’ve just lost 50-75 pounds on my max deadlift while only gaining 25 on my squat. For another example, Reed from these forums is a super-squat dude that doesn’t really train the deadlift; he squats literally twice what I can squat, and we had similar deadlifts.
I don’t think the deadlift really drives the squat either, though. It’s perfectly fair to point out that squatting in the low/mid-400’s with a 600+ deadlift is a huge imbalance the other way. I liked it more - it was cool to say that I could deadlift 600+ pounds, and I honestly did feel better - but it’s not like the big deadlift was getting my a big squat all by itself.
So the answer is probably that, derp, if you wanna have a big squat and a big deadlift, you’ve gotta put in the effort on both. If you only have limited time and energy (which has always been my whole deal, trying to get decent strength out of a minimal effective dose) then my anecdotal experience suggests I personally preferred more energy on the deadlift and just kinda living with whatever I could get out of my squat. YMMV.