The advantage of the dual bodypack is that you have a full backup of everything ready to go. Backup mic, backup transmitter, etc. You could accomplish almost the same thing by simply using 2 of any wireless system and just having 2 channels on your mixer and manually turning on one if the other fails. I'm not sure if the auto failover feature would work on the ULX-D system if say the mic were to come unplugged for some reason.
I've been using the QLX-D systems which is the baby to the ULX-D. When using a pair of Audio Technica ATW-A49 antennas (way cheaper than the Shure counterparts...) I've never had a dropout concern, even with occasionally using them at distances over 500' outdoors. Usually, I just end up mounting the paddles on stands right next to the receivers with 15' patch cables.
Now, that's working in relatively clear RF space. If you're in a congested RF area, then the rules change a bit, and getting the antenna as close as possible can help. If you're concerned that someone in the next room might turn on another transmitter in the same frequency space, having the 2nd bodypack on a separate frequency is going to give you the ability to keep functioning.