Ron--
With two roasts you could go either way, high heat or low/slow, pulling as each hits your target internal.
The dynamics of the lower rack are different when there is no water in the pan. Regardless, finish time is more a factor of overall roast size--thickness primarily--so if their sizes are similar in that way then they should finish fairly close to each other. Variances in heat at each level can throw that out of whack--it's not often by much--but if you're able to track internals of both you can always switch positions, if needed, somewhere bewtween half and three-quarters of the way through the cook.
When I do high heat cooks I most often start at a much higher temp at the outset for the sear phase and then lower the heat to ~350 for the duration. A straight 350 cook does not always need a finishing sear--much depends on the temp of the meat when it went into the cooker, how stable the temp was during the cook, and what the components of the rub are. I usually just gauge it by look shortly before the roast hits my target internal. Often it's good enough already. If a sear is required I do so over direct coals or, occasionally, get my oven up to temp in the house; the former more often as the oven is usually occupied.
