I have always assumed that running both elements would destroy the bulb, but wasn't sure, so I asked the manufacturer, here's their response;
Hello Kinarfi,
Thank you for your inquiry. Halogen headlamps are not designed to support the heat generated by continuous dual filament operation and will fail prematurely. The heat generated by dual filament operation can also damage the lamp wiring harness and lens reflector assembly.
Also, it is illegal to energize both the low beam and high beam filaments simultaneously for an extended period of time. This method of operation is specifically prohibited under federal motor vehicle rules of operation. (Reference; FMVSS 108 ). The rules do allow for a momentary overlap to prevent a 'dark out' period when switching from low beam to high beam and back.
The following paragraph was copied from FMVSS108:
S5.5 Special wiring requirements.
S.5.5.1 Each vehicle shall have a means of switching between lower and upper beams that conforms to SAE Recommended Practice J564a Headlamp Beam Switching, April 1964 or to SAE Recommended Practice J565b, Semi- Automatic Headlamp Beam Switching Devices, February 1969. Except as provided in S5.5.8, the lower and upper beams shall not be energized simultaneously except momentarily for temporary signaling purposes or during switching between beams.
Sincerely,
Jordan Brown
Consumer Care Specialist
OSRAM SYLVANIA, Inc.
Kinarfi