It ain't rocket science, I know that ain't what you want to hear, but a vent is a vent, add a "Y" connector to your hose and you are done. If you want to do complex formulas out of the book, you probably can, but when you get to the store they will probably only carry 4 inch stuff so all you calculations will be a waste of time. If you get lucky the store will have 4 inch and 6 inch stuff and then you can double your choices. In the world of HVAC that is what you are going to find, unless you are a certified dealer and ordering in company specific stuff.

If you know absolutely nothing and need the basics go and get a time life book on plumbing or borrow one from the library.