I LOVE upper/lower splits. I've had my best results from them. As for overtraining, I've been able to do 2 lower body days a week while also doing running and sprinting sessions throughout the week, while still progressing on my maximal lower body strength.
When I plan my training I give one lower body day to squats, and one to dead lifts. I work up to heavy singles, doubles, or triples in whichever lift I'm focussing on that day, then do some other lifts to finish myself off. My favorites are high rep leg presses, SLDL's, leg curls and calf presses.
I also like to use partial versions of squats and DL's, to keep things interesting and let me move some serious weights.
On my upper body days I like to superset pushing and pulling movements, but I still keep things heavy. I choose my first movement based on whatever I need to improve the most. If it's shoulders, I'll start with on overhead press and superset with a pull up or a row variation. If I need to focus on chest, I'll start with a bench press variation.
Don't worry about overtraining your back. It's practically impossible, and most people drastically undertrain everything on the back of their bodies.
Current PRs at 242- 495, 345, 585, 1425