Yes you can use multiple teensies !
The bigger test I did with teensy is 10 meters of 144 pixels/meter RGB at 60 FPS (from Modul8 Syphon to the LED, all 60 FPS). So it's 1440 RGB Leds, so 4320 values 60 times per second, so 258 kbps. So it's far below usb speed of teensy (on Teensy 3.1, USB always runs at 12 Mbit/sec)
I don't remember exactly, but at some point FPS dropped, I don't know if that's because of the Fast LED configuration. I know I had reduced to 2 MHz the APA102 here:
FastLED.addLeds<APA102, DATA_PIN_LINE0, CLOCK_PIN_LINE0, RGB, DATA_RATE_MHZ(2)>(leds_line0, NUM_LEDS_LINE0);
Maybe spreading over multiple teensy pins would have solved.
I don't remember exactly why. I think at 12 MHz I had a issue.
Anyway, for a big setup, and in a lot of configuration, a LED controller (like Advatek Pixelite) is better. And you don't risk to burn your USB port if you plug the wrong cable at the wrong place...