How does data rate change when Wifi channels overlap?
27Mbps each? You wish!

For starters, bear in mind that routers don't simply "stream" data like a TV transmitter, they just provide a path for other devices to carry out two way communication using higher levels of protocol such as FTP or HTTP that are in turn layered on top of TCP or UDP on top of the wireless frames. That all requires various levels of protocol management traffic to flow from the receiving interfaces and applications to the sending interfaces and applications, and with Wi-fi being half duplex that all eats into the maximum data rate. Add on the various protocol overheads and some hardware-dependent overheads for encryption and it's generally reckoned that you can't expect much more than about 20Mbps of actual data throughput if you have exclusive use of a channel.

Now add a second pair of devices on the same channel and factor in the random delays added by the CSMA/CA algorithms to avoid collisions.  Although it's impossible to give a definitive answer when there are so many factors involved, I don't think you could reasonably expect more than about 6 or 7Mbps of data throughput for each pair.