You can compile the Teensy to act as a USB Keyboard on your PC and send key messages with the keys of your choice such as the CTRL keys. You can find all the details here:
The connection of a closing button or morse paddle is not a problem using whatever jack suits your application.
With some experimenting of time limits together with debouncing it should be possible to get a robust interpretation of long and short I would assume.
First start with the Buttons example. In Arduino IDE with Teensy selected, click File > Examples > _Teensy > USB_Keyboard > Buttons.
This example does more than you need, using 10 buttons. Run it first and make sure it works properly.
Once you have the Buttons example working, deleted 8 of the buttons and test again.
When you have it down to only 2 buttons, using the info from that page (scroll down to "Individual Key Press / Release") to replace the Keyboard.println() stuff with Keyboard.press() and Keyboard.release().
Should be pretty easy. The most important part is to start with the known good example and take small steps depending on your skill and comfort level with programming.