My house is roughly 60 years old. The kitchen sink drain was never vented, it is approx 2" pipe that runs straight into the floor no vent going up. The sink drains fine, never a problem. I want to put a bar sink in my basement and this is the only drain I can tap into. It would be approx 20 feet away. Since the drain is not vented I think the bar sink would vacuum out the kitchen sink trap above it. I have no way of venting the drain through the roof. Is there a way using an AAV to vent both sinks? Can I tap the bar sink into this drain and put a tee with an AAV above to vent both sinks? Would an AAV need to be above the kitchen trap? Would I need to use 2 AAVs one for eack sink, near each trap? Or should I scrap the idea of the bar sink altogether?
Any help would be greatly appreciated.

As a rule I would suggest that you run a vent on the kitchen sink. This would be good plumbing advice, however, since the sink has been draining with no problems and running a outside or roof vent would involve labor and expense I would leave well enough alone.

You may connect the bar sink to the kitchen drain line but you'll have to install a AAV after the bar sink trap,(see image). This will vent the bar sink without affecting the kitchen sink. Good luck, Tom

That was a great help. Thanks
Should I add an AAV to the kitchen sink under the counter also to be on the safe side?
Or just leave it be?

That was a great help. Thanks
Should I add an AAV to the kitchen sink under the counter also to be on the safe side?
Or just leave it be?
Unless you wish to reconfigure the kitchen sink drainage to accept a "P" trap I would leave it alone. Regards, Tom