Outdoor Sink Drain/Vent Question

Have a problem.

I am installing a summer kitchen on my Patio and there is existing 3 or 4 inch plumbing drain going straight down into the foundation with a cap on top. I have no Idea if this is tied into the roof drain vent but I do not believe it is unless its tied in under the foundation.

I am installing a dual sink, There is a adapter on the existing 3in drain to feed the drain from the sinks into.

I want to incorporate a drain valve in my installation.

I was thinking for ease of installation i could use a Studor Maxi vent on the existing drain pipe as it has a cap on top of the drain going into the floor. However If this drain is connected to the roof vent could that be to much air being let in and cause drain problems.

http://www.plumbingsupply.com/autovent.html

OR

Use a small vent After the trap prior to the drain

Another option i was toying around with was bringing both sink drains to a common 4 way prior to the trap and installing a vent there?

I would put a picture here but i can’t figure out how.?