Use the same saddle you use on your regular solo bike; however, just recognize that you probably have two things working against you on the tandem with regard to comfort:
1. If the rear cockpit of the tandem does not allow you to replicate your regular solo bike riding position (saddle set-back, bar height, and reach), you will end up with a different hand/butt weight distribution that will most likely put more weight on your sit bones AND will also have you sitting more upright. This will create new "sore spots" that will eventually go away the more you ride the tandem.
2. The nature of tandem riding is such that most teams will not ride out of the saddle (e.g., on small climbs, etc..) nearly as often as they do on their solo bikes. These nomal out of the saddle stints on solo bikes go a long way towards relieving saddle numbness and letting the chamois pull moisture away from your skin. The "fix" is something we refer to as "butt breaks", which is quite simply making a conscious effort to ride out of the saddle on your tandem at least once every 15 minutes or so. In time, teams who race will usually figure out the whole standing thing and will begin to stand much more often as a natural part of riding.
If you still find that you're having saddle sores... use more Chamois creame if it's chaffing soreness or give another saddle design a try, tailored towards fixing what hurts. If it's sit bones, figure out if you need a wider or more narrow saddle. If it's the soft tissues in your crotch, try a saddle with an anotomic cut-out.