I've often heard the statement that "whatever direction your relationship is going, a tandem will accelerate it." I like the way the idea was expressed today by Scott Martin in the "Scott's Spin" section of the RBR e-mail newsletter:
"Then there's the tandem. When you're single it's a dandy dating vehicle. When you're partnered up, the unique communication challenges of a tandem will (a) strengthen a solid relationship, or (b) pulverize a shaky one. Either way, a good thing."