You used to be able to use a [br] or [br /] tag, but those seem to be disabled since the election day hack.
Perhaps using back to back [blockquote] tags would work - although the signature might be indented a few characters. Like this:
[blockquote]line 1[/blockquote][blockquote]line 2[/blockquote]
Unfortunately that uses a lot of the limited number of available characters for a signature here. If I remember, I'll try to play with some ideas this weekend.