Upper/Lower splits are what you get with training systems similar to the conjugate method/westside. Hard to argue with the effectiveness of those. If I were you, I'd try to get away from thinking in terms of body parts as from a planning aspect, it just complicates things. Deadlifts work upper, mid, lower back, but also glutes, hamstrings, abs, to a smaller extend, quads. Front squats rely heavily on quads, glutes, lower, mid, and upper back strength. As you can see a ton of overlap and thats just from two lifts... one considered a "back" lift and the other a "leg" lift.
1) Thinking in terms of upper/lower... deadlifts and squat work on the lower, everything upper body... on the upper body day. For the lower specifically, you can squat and dead on both days or you can squat on one, deadlift on the other. Many people on this forum are squatting 3 times a week and on top of some type of pull from the floor. Past this forum, many people squat 4,5,6 times a week... Its possible to do it more than once.
2) This was just answered, but treat the deadlift like a lower body lift. Back work such as rows and chins as upper body stuff.