There are potential issues with proper lubrication. More importantly you'd be driving forward by applying torque to the weaker coast side of the gear set.
Unless you need a high pinion in the rear for serious rock climbing and/or racing, namely where you don't mind spending a sponsor's money on broken parts, just stick the high pinion diff with reverse cut gear set in the front - where it belongs - and a low pinion in the rear. That way neither diff will be running on the weaker coast side of the gear set.
