My basement is finished w/ 1/2 bath & wet bar. Bathroom sink drains to the rough-in meant for shower drain (long story). On other side of wall, wet bar drains into the same drain as the bathroom sink. Both sinks have their own p-traps (plus the rough-in trap under concrete), but neither has dedicated vent. How should these 2 sinks been drained/vented properly w/ only 1 rough-in drain available? Any good reference drawings I could look up?

Both sinks have their own p-traps (plus the rough-in trap under concrete)

For starters, THAT is very illegal. You may not double-trap a fixture; once the drain forms a p-trap, from that point on it has to continue straight until the sewer - it may NOT pass through another p-trap along the way. Sorry!

My basement is finished w/ 1/2 bath & wet bar. Bathroom sink drains to the rough-in meant for shower drain (long story). On other side of wall, wet bar drains into the same drain as the bathroom sink. Both sinks have their own p-traps (plus the rough-in trap under concrete), but neither has dedicated vent. How should these 2 sinks been drained/vented properly w/ only 1 rough-in drain available? Any good reference drawings I could look up?

OK Jim, Tell me the long story. How does one sink connect into a common stubout across the room from another sink? Moshie was bang on when he told you it was against code to double trap a fixture. You not only double trapped the sinks but failed to vent them. Double trapped and unvented fixtures are not only illegal but present a health hazard to you and your family. The reason being that the vents relieve the suction caused by draining water. This suction, if not relieved, will vent through a fixtures water trap seal pulling it back into the drainage system and opening a path for sewer gas to escape into your home. Sewer gas is a danger to your families health and the methane comtent makes it explosive.
You should break up the cement, remove the old shower trap and extend the drain over to where you wish to install the bar sink. You can then turn up with an short sweep to a 2X2X 1 1/2" drainage tee 17 1/2" off the floor to the branch of the tee where it will stubout of the wall at 1 1/2". You will then run a vent or AAV (cheater vent)off the top of the tee. That's for hooking up the bar sink or lavatory. Since I have no clue as to how the other fixture is located in relation to the one I advised you on or how you had it connected to the other drain I can only advise you to rough in another drain and reventback into fixture number one.
Jim, don't let this slide. Redo your project safe, legal and correct. Good luck, Tom