I have only done it once but the baby backs in my experience will be done 45 min to an hour earlier. I guess you can put the spares on first. Let us know how it worked out.
Ron as Tony R states you put the spares on first. I do this all the time when cooking ribs and chicken. Put rips on first, then add the chicken. In your case put spares on first then add the backs.
What would happen if you put them on the same time and when the Backs are ready, just foil and set aside and then put back on grill when Spares are on the sauce/set mode. Thoughts?`
