Two Bluetooth devices + MacBook Pro + 2.4gHz WiFi = no network throughput; why? My wife has two MacBook Pros.  One is old; the other is new as of a week or two ago.

We have a combination of three Bluetooth devices: two keyboards (one Apple; one Logitech) and a magic mouse.

We have a Comcast-supplied barely polished turd of a combination 2.4gHz 802.11n WiFi gateway/router (standard XFinity fare).

The problem: any combination of two or more Bluetooth devices paired to either machine causes network throughput to drop to effectively zero.

This is true regardless of WiFi channel and is always reproducible.

This is not a pairing issue—that is, it's not the case that the keyboard is actually paired with some other machine; we've tested this with only one machine powered up at a time.

This is not a machine per se issue; the old MacBook Pro is from circa 2010; the new one is from 2014.

This is not an OS issue; both MacBook Pros are running fully updated versions of Mavericks.

Our iMac from late 2013 has two Bluetooth devices paired with it just fine.  It is closer to the router in case that has any bearing.

Wireless Diagnostics shows an "Excellent" signal-to-noise ratio throughout on all machines in our household.

Any tips or pointers?  I've read the threads on generic Bluetooth interference, but this seems very specific to that second device being switched on, and very specific to something that two MacBook Pros from different years must share in common.
I had the same issue. Did a bunch of troubleshooting with Apple over the phone and in store. The final diagnosis was a logic board and wireless card replacement.

When they took it in to repair it, the tech noticed only one of the three antennas were connected to the motherboard. His theory is that the one antenna was enough to support a connection or two but became overloaded in certain situations, very easily. After reconnecting them all has been working well.

So my suggestion to you is to either open it up yourself or bring it to Apple and see if that may be the issue.