Stacked tube blow back...There is the bolt riding on the top, while a hammer on the bottom hits the valve which opens air up, while goes up through the bolt which fire a paintball.
Tippmanns are just blow backs. They do not have 2 stacked tubes.
As for not liking to upgrade them, it's all perference. A spyder is a spyder no matter how much work you put into it. Let it be $10 for $1000. It will never shoot as nice as a Timmy or anyother high end gun. Don't listen to anybody if they tell you elsewise.
I had a spyder with big dreams of up grading it. I got a new barrel, a T-board and a Halo. Along with a compressed air tank. 2 of those 4 upgrades were generic and I can use them on my matrix now.
Feel free to hit me up on AIM: nypa8ntballa14 if you have anymore questions.
*EDIT*
There you can see how there are 2 "stacked tubes"