the sound has a lot to do with length, diameter and bends of the tube, plus the muffler/cats used. True duals are tricky, you'll have to get a dual-outlet header. I don't know offhand who (if anyone) makes one for the 1990 motors. Some people just split the pipe and run two sections from the single downpipe, so this isn't a true dual exhaust. There are a lot of things to consider.
At any rate, if you want to keep the engine power, you'll need to run smaller pipes if you run duals. The 4.0 motor doesn't have enough airflow to run dual 2.25" or 3" pipes so you'll actually lose power and torque if you run dual large pipes. If you're just looking for looks and sound, go right ahead otherwise you'll have to do some math depending on if you're running true duals or splitting the stock exhaust.