No! Not necessary...I've run pit, mag, abx, insulin and IV push meds in one line!! I always call pharmacy to make sure the meds are all comparable and make a note in the chart verifying that I called and who confirmed the safety. Running two lines is unnecessary stress to the patient...IMHO. :-)
Just my 0.02 but if I have a patient sick enough to be on Mag I like a 2nd IV. If you're running several things simultaneously it's easier to run two pumps--or two double pumps!-and keep track of your fluids that way. It does suck for the Moms but so do seizures etc! I also think running things like oxytocin and mag in the same line--even piggybacked at different sites--is a much higher risk for a medication error--ie after delivery, bolusing with mag instead of pit.