Neat idea, bit I think that adding so much weight to an outrigger (that weight being your daughter-not to say that she's overwieght, though:) would be damaging to the boat and probably unsafe. The usual outrigger is generally very lightweight, and the supports are only meant to take loads that are perpendicular to the longitudinal axis of the boat. But what happens when one of you gets hung up on a rock or something? You'll end up making the rectangle formed by the two kayaks and two supports into a parallelogram. Even going at a 45-deg. (or so) angle to waves would cause some interesting bending stresses as one boat is flat in the trough of a wave while the other is parked at an angle on the wave's face.

Towing is probably safer, although not as condusive to the father-daughter-bond-making you're probably looking for. If you go this route, add a short piece of strong (i.e. not deck-line) shock-cord to one end of the tow line -- it will help buffer the push-me-pull-you effect, especially in waves.

As to the stability of your daughter's kayak, how about adding Sea Wing sponsons (air tubes on either side of the kayak) or a down-sized version of a pair of outriggers, maybe sticking out just one foot on either side? You could mount a couple of t-nuts to the aft deck of her kayak and bolt or unbolt the mini-outriggers as needed. If you wanted to go the cheap, fast route, rather than stitch-and-glue a pair of outriggers, how about using a foam Pool Noodle cut in half, one for each side. Or you could glue together a few layers of R-board or some other foam and shape a more efficient hull-type shape.

Hope this helps... Dean