The best way would be directly connecting Arduino Uno's RX & TX pins (pin 0 and 1) to Teensy++ 2.0's RX & TX (pin 2 & 3).
In other words, connect Uno pin 0 to Teensy++ pin 3. Connect Uno pin 1 to Teensy++ pin 2. Of course, also connect their grounds together.
On Uno, use Serial, and on Teensy++, use Serial1.
For the sake of development, you might want to connect the 2 signal wires through a double-pole switch (or 2 single-pole switches), so you can disconnect the Teensy++ when you upload new code to the Uno board.
On Teensy++, the USB is completely separate from pins 2 and 3, so you can upload new code to Teensy++ even when Uno is still connected.