Originally Posted by Winger CT
No idea how Co2 would behave...
As long as you're not mixing gas sources, just fine.
It was actually fairly common in the early-mid '90s to see players with Autocockers or Tippmann Mini-Lites run dual tanks with a vertical 3.5oz and a 20oz on bottomline. The smaller tank acted as both an expansion chamber and condensation reservoir to help keep liquid CO2 out of the gun.
Some VM-68 users even ran triple tanks to feed their thirsty guns.
Picture from VM-68.com. http://www.vm68.com/vm68/customvms10.html