Here's a stupid suggestion from a guy who can't make a scarf joint. Try making the paddle in two pieces. Make the handles (loom) too long. tape them together, overlapping the ends. Paddle with this, adjust the tape (pretty high tech stuff - write if you need help) until it suits you. Cut the shafts a bit long and make a scarf joint.
I word of advice here. When complete and done, you will like the handles much better if they are not round, but oval shaped. They should be longer pointing fore and aft so as to fit comfortably into your thumb and fingers. Easier, maybe, if you start with it round, and use a plane and sander to flatten it a bit, try it, flatten some more, etc.
If you haven't paddled with a double ender, here is some more advice. (Worth every penny I'm charging you, too.) Most strokes will be made holding the paddle symmetrically, with the same length sticking out to the left as to the right. When a bait veers or needs a nudge in a direction, slide the paddle to one side and hole it asymmetrically. You can even cup a hand around the very end of a blade and hole the other had wherever it is comfortable, with a LOT of paddle out to one side. Also, learn to do both the bow rudder and a stern rudder strokes. Invaluable in tight turns, going into eddies, landing along side a bank or dock, and impressing hot chicks.