@srleone wrote:
.... a double line linetype that I can share within a .lin file. ... draw a single line, but have it appear as two parallel lines.
The only way I can think of to do that is with a linetype involving a Shape or a Text character. Here's an = sign in the TXT.shx font (above), and a series of them, slightly overlapping (below), that could be accomplished in a linetype definition:

But curves would be kinky:

And things like changes in direction in Polylines would not look as I expect you would want them to:

A Multiline Style would overcome the latter, but [as pointed out already] curves would be out of the question, unless drawn kinky in segments [but the segment joints would at least be cleaner than the above].
Kent Cooper, AIA