I'm afraid it makes no difference. I tested with one job running and get the same result. In fact I have noted that each backup job tends to max out at approximately the same transfer rate (ie if I start one job and let it settle at whatever low speed, and then start a second, the first job doesn't suddenly half it's throughput, rather the second tends to match the first), which doesn't come close to causing the drives to thrash. In fact drive activity is barely registering during these backup jobs (sort of pulsing activity rather than any sort of continuous read/writes).
I hate to keep referring back to a different piece of software, but the Kopia backup jobs I set up were able to complete their runs inside of an hour, maxing out the transfer link even while running at the same time as the urbackup jobs... And those are file level backups which as Binhex mentioned should tend to be slower based on windows file system reads.