I'm going to install a second stock replacement (both duralast T5-DLG) and I was wondering how I would vent both of them.

I was thinking a Y adapter for the stock vent hose, or making a new hole and venting the second battery separately.

Any ideas?

 

Either way you are thinking will work. :)

 

Does it matter what kind of hose I use, or how big/thick, etc...?

I'm having trouble finding parts to do the Y idea.

 

Any autoparts store will set you up with the right stuff. Dorman and many other parts manufacturers make kits and parts. Just visit your autoparts store.