SpudFiles

I have a couple used propane canisters like the one in the pic below, neither of which is big enough for what I want to do, but both combined would work well. Can I just connect the two with a Tee? What are the cons to a set up like this?

It depends what you are using it for. What are your intentions?

It really depends what you are using them for, are you refilling them with propane or are they some kind of air storage tank, if so there could be better alternatives.
EDIT: gun freak beat me to it ...

Due to the way they are built, they are not able to provide high flow rates unless modified. The opening into the tank under the schrader valve is less than 1/8 inch in diameter.

Sorry, not enough detail - I want to use these for air canisters. I'm having trouble finding a fire extinguisher bottle, so I thought this would be the next best option. But I want to know the down side to two vs one canister.

Technician1002 wrote:Due to the way they are built, they are not able to provide high flow rates unless modified. The opening into the tank under the schrader valve is less than 1/8 inch in diameter.

According to this How-To, I thought I could drill to opening out a bit (~5/8"), which is bigger than the orifice in my valve. So I hoped it would work. Am I misunderstanding something?

Drilling it out is highly recommended for the flow.

What size barrel are you intending to use? I drilled one out and built in my own high flow valve.

You many recognize this tank from the other thread.

This is damage from a single propane bottle and a D cell at 100 PSI.

First of all, there is nothing that isn't totally awesome about that photo. I read your post about that cannon - very impressive.

I built one of the piston QEVs i found on here (finally found a solution to the hose barb needed - I used a pipe nipple instead) The smallest opening in there is about 1/2". I want to make two barrels that are interchangeable. I'm thinking marble size(~.5") and golf ball/potatoe size(~1.5"). I know the 1.5" barrel is probably going to compromise performance a bit, but this is my first cannon, so this is more about the build and experience than performance.

I attached a pic of my set up to this post. I haven't finished the piston yet. Any tips/critiques welcome.

A single bottle with as large as an opening as you can make will work best. Adding the plumbing to add two bottles will lengthen the pipe between the bottle and barrel and add more turns and fittings to cut the flow rate. You want as little plumbing as possible between the chamber and the barrel and it should be as large as possible.