Is there a way to keep the headlights on, while the brights are flashed on?

Um... what ?

You don't mean burn both filaments do you ?

Heya Ian

That's precisely what I mean. I would use it only in Off Road situations, never on public DOT approved roads.

Would it be damaging to the truck's electrical system? Do both filaments share a common ground?

I would think it would cause the bulbs to burn out quicker.

Get a nice set of off road lights instead, they'll put out much more light in a much better pattern.

Both filaments do share a common ground. Many people have melted their headlight sockets with high wattage bulbs. I'd bet good money that turning on both filaments at once would do the same.