yeah they drain perfectly! it drains the same as a circular drain.
It is the drain.
Sinks and tubs must drain to the septic tank.
drain
The water goes down the plughole/drain and then down the pipes.
a 3 inch pipe
well kitchen sinks are set at 2 fixture units and on a 2" drain you can have 4 because horizontal 2" line can have 8 fixtures units and 2" vertical can have 16 fixture units.
Unclog drain that both sinks drain into.
Yes, they work in either.
The drain pipe should have a vent that goes up where the sink drain goes into it. That is where it vents. Sinks drain because there is nothing causing a vacuum when the water leaves the sink.
The trap and drain in most sinks gets it's venting from the overflow tube in the sink or in the case of a double kitchen sink, one side gets it from the other. One problem is with most cultured marble and cast sinks. Most of these do not have an overflow feature and may drain slow.
You should get your drain lines snaked
If you go to http://home.howstuffworks.com/home-improvement/plumbing/how-to-fix-sinks-tubs-and-drains.htm you can learn how to fix your drain. They also teach you how to fix other common problems.