Originally Posted by 2Rodies
Is this normal due to the long leangth of the cable.
Yes, this is a normal and accurate first impression. You have twice as much cable and at least twice as much cable stretch.
(Or -- with a nod to Scott -- to be more accurate, you end up with at least twice as much loss of efficiency in the "system". More specifically, on a single bike you lose cable-pull efficiency by a factor of X due to various things. On a tandem, the factor is ~2x. At the same time, you are exacerbating 2x because you are riding a bicycle that weighs twice as much and are pulling on the brake lever 2x as hard in an effort to achieve the same result. When you combine all of those things, you end up with a rear brake that feels "mushy" and would feel mushy even compared to the same brake device on a single bike.)
Originally Posted by 2Rodies
Would the Avid shorties increase or decrease this feeling?
As Retro noted, I'd probably need to see a photo of your current brake set-up before making any specific recommendations on how or what to tweak to achieve what result. As you can see from some of the other responses, there are quite a few different ways to "firm up" a canti or linear pull brake, some are merely adjustments to reduce cable travel along the run or at the brake, others involve fitting different brake pads to improve the "bite", brake "boosters" to neutralize flexing at the seat stays or forks, or different models of brakes that may or may not have more inherent flex. However, underlying all of this needs to be cost-effectiveness. After all, once you have enough brake leverage to lock the rear wheel, you have "enough". Any more than that might give you some marginal improvement in brake lever "feel" but it won't make your tandem stop any more efficiently as that remains the domain of the front brake.
Just my .02.