Hi Gary and welcome to the forum. I also have a tandem (Oasis) and sail it solo occasionally. I agree with Stobbo -- ballast forward is essential, as is the large (sailing) rudder if you have the twist and stow rudder. I also use Turbofins which double as a centerboard of sorts, but plug or tape over the front drivewell to streamline the hull a bit.
Both the Oasis and Outfitter are very stable with the Hobie sail and I think you'll be comfortable without Sidekicks with these beamy boats. Your ballast also lends stability by lowering the center of gravity. You can always get a Sidekick later if you think you need it.
For ballast you can carry bags of water -- fill em right on the spot and throw them forward in the front cockpit. I use lead weights -- one in the front drivewell and one in the front hatch. The lead lets me use less weight, keeping it lower and more forward. The disadvantage is having to transport it. Anyway, 50 or 60 lbs will probably do it for you.
Here's a couple of pictures of the Oasis sailing:
There are lots of tips here on rigging improvements. To start with, the most important thing is a small pulley or micro-block to run the mainsheet through at the aft padeye:
This improves the speed and handling of your sail, especially in gusty winds (get a simple block, not a swivel block).
Tandems are not the fastest sailers, but sailing them is a lot of fun anyway!