I've never seen 1/4" EMT, 3/8" flex armor cable yes. I would suggest you run 1/2" EMT about 2-4" pararell to the 3/4" pipe to a 4x4 junction box and use that as your box for camera #1 and the connection point for another 1/2" conduit running to camera #2. For camera #2, you can use flex steel cable or liquid tight non metalic if you don't want to pipe a long distance..looks tacky using flex but the job still works just fine. You can however use flex tubing/conduit for your whole home run if you wish..looks like crap though but it would work. Putting the low volt and regular 120 in the same pipe may cause some issues.