The best way would be to drop the idea of a body-part split and focus on training the movements instead of certain muscle groups. This way you can train squat and deadlift more frequently and promote faster strength adaptations and improve technique.
If you aren't very strong at these movements or are new to doing them then I would train them as frequently as possible. If you are decently strong then I would keep frequency moderate rather than high because weekly volume is probably going to play a larger role in your progress at that point. That is just how I would handle it personally, that last part is kind of subjective and simply my preference.
Personally, every 3-4 days is the minimum I train a movement, and that is my preference for an intermediate / advanced lifter. Beginners can benefit from even higher frequency.
I've had lower back issues - I think from having
not exercised seriously for several years after marriage, possibly originating back in my army days with all the idiotic schlepping around with massive amounts of gear on my back. Then I got back into it, and have focused heavily on core exercises for the past 3 years, pretty solid outer and inner core now.
So that's the background - and I'm 52 now btw - and the other day I finally gave squats a try - felt inspired after watching a bunch of Rippetoe videos.
So I did the whole routine - started with empty bar squats, loaded up one set of 45s, did 10 reps that felt good, loaded up another set of plates, another 10 reps, still feeling good, and took it to 275 in the final set, also with 10 or maybe 8 reps.
Happily my back felt just fine the following day.
I'm not planning on getting into deadlifts any time soon (my hand grip strength needs improvement, and using straps seems to defeat the purpose) I think that's asking for trouble, but if I can handle squats I plan on incorporating that. I've only done machine work for legs up until now, but I'd rather do full body movements.
My goal is to keep strengthening the core, not to build massive legs - jeans are already hard to come by. So if I can get to 350lbs for at least 6 reps, that's a good level for me. Not interested in 1RM at all, I'd rather avoid injuries, a big factor to my training approach.
So here's the question, how complete of a leg workout do you think squats are? For example, I like weighted dips, I think they do a great job of engaging numerous muscles. Would you rate squats as the equivalent for legs/core?
And considering my age (52), do you still suggest that many leg days per week? I hadn't planned on doing squats more than twice per week,and go heavy enough to be able to maintain at least 6 reps per set.
Edit: re-read your post, and I saw that you're not suggesting 3-4 times per week, but at least every 3-4 days. So, maybe twice per week for me is a suitable compromise, considering my age?