If you're doing comps, I wouldn't let the top ribs drip on the bottom ribs as that will mar the perfect bark finish you're striving for your bottom racks. Instead I would only load the bottom rack with meat (any kind) once bark forms on my top grate ribs (usually about 3 hours @ 275 degrees).
That's the rational why I put ribs at 6:30 am on the top rack and chicken on the bottom at 9:30 am. By 9:30, the bark for ribs have already set and will not drip on my chicken.