The purpose of a relief well is to intersect the original well bore and provide an alternate route to let the pressure bleed back so that you can control or correct the damage in the original well. In order to be able to drill a relief well, you have to know the exact trajectory and orientation of the first well. There is an advantage to drilling both of them simultaneously, but you're still going to lose time by having to find the exact trajectory. Relief wells are not always successful in finding that exact trajectory.
If you picture it, you've got a conduit that's 12,000 or 18,000 feet long, with a diameter of 8.5 inches; you're a mile away, or a kilometre away, and you're trying to drill another 18,000-foot hole to find that line. You have to do a lot of pre-work first to plot out that trajectory. It's not always as effective to do them together; they have to be staggered in order to do the analysis.