Welcome goalie_trainer,
You can most certainly accomplish your goal of launching Pseudo-Corpse Canisters up to several hundred yards. It can be done for very cheap as well, but this also depends on what exactly you're looking for, and what type of performance you require.
The answer to your question is yes, it is possible to assemble a dual valve cannon as you describe. however, since you hope to keep the launcher as small as possible this wouldn't be necessary, and would halve the cost if constructed with only one valve.
In order for me to assist you further you'll need to answer a few questions.
1. Any constraints? (time, money, available materials..)
2. Will the launcher be handheld?
3. What are the the approximate dimensions of your proposed projectile? (size, mass..)
4. What are the distances you are hoping to achieve the launcher?
5. In what way do you plan to pressurize the launcher?
6. Must the cannon be portable? (ties in with #5)
7. Lastly, but most certainly not least, have you checked with the your state laws regarding the legality of potato guns? (assuming you live in the USA).
Is this the type of launcher you had in mind?
Link. Of course there are endless variations of this type of launcher but it's a proven design and can be constructed easily.
Thats all I can think to ask at this hour of the night

But it will help narrow your scope, and remember, were here to help!
In the meantime, be sure to check out the
Spudfiles Wiki, it should answer some of your questions.
Good luck, hope to hear back from you,
TaTer