> 1. She does not have the time (or interest) to really practice in rough
> water conditions which would greatly limit our ability to do coastal ocean
> trips.

You still will not be able to handle rough water conditions, because one side of the cat will have a skilled paddler, and the other will not. Not to mention the mechanical problems you would have trying to turn the cat into the waves with just you on one side.

> 2. Our paddling skills are very different. I'd have to wait for her, or
> build a double - which is relatively useless for single use.

You will still have to wait for her. Because if you overpaddle her, you'll just turn the cat.

> 3. Medium size dog (good swimmer) and small baby (not old enough yet, but
> should be in a few yrs) will both want to come along. Linked kayaks would
> provide a much more stable platform.

> 4. If I ever get around to trying kayak sailing this would be a great
> platform for 2.

Overall I like the idea, but not as a cure all for balancing skill. It would be stable, and bulky. Which could work out very well, especially if you rig up a cloth deck between the spars like a Hobbie.