You should definitely be able to select more than one certificate.
If this is a manual setup, I would untick "Verify the server's identity", and then get that working.
Once you have that base working, come back and add certificate validation.
Also note that I can only use "Simple certificate selection" without additional config maybe 50% of the time. If you only have one certificate in your user store - then no problems. Otherwise, you might need to add certificate selection criteria to make it choose the correct certificate.