How can I connect two plumbing vents in a horizontal run?
The product you want is called a Vent Tee and they are made in PVC.  If they're not stocked at your local home improvement stores, check a specialty supplier (locally or online) and you'll definitely find them.
Keep in mind the vent needs to be able to drain water (condensation, rain, etc.)  Horizontal vent runs should follow the related minimum slope requirement -- usually 1/4-inch per foot, like drains.