Hey thinblueline,

You can also just rent a solo if you don't have room to store one.  Sounds like it would be a somewhat rare occasion where you'd really need it anyway.

As far as fishing from a tandem as a solo paddler because your partner doesn't want to go out, it certainly can be done.  As db noted, having some ballast up front helps a lot and your safest option is the bag of water.  I've used my 5-gallon collapsible water jug from camp for that purpose pretty successfully.  I used to use rocks until someone from this site pointed out the obvious flaw in that plan.  Doh!!