The Bluebook specifically says to defer to the Chicago Manual of Style where the Bluebook does not prescribe a rule.
Chicago requires 1 space.
Also, the Bluebook is at most followed only in spirit, more commonly in the breach. Most of it is specifically for "scholarly" articles, not practitioners' documents. Smallcaps is an important part of following Bluebook for scholarship! There is a slim section on how to translate requirements to briefs, but it is underspecified so lawyers just kind of muddle along. All while insisting they follow it to the letter. They don't.
Personally, I'm not familiar with any double-space rule. I simply mentioned the Bluebook in response to the above poster's misleading argument that a legal convention is "dead" on the basis of style guides that aren't generally relevant to lawyers.
Chicago requires 1 space.
Also, the Bluebook is at most followed only in spirit, more commonly in the breach. Most of it is specifically for "scholarly" articles, not practitioners' documents. Smallcaps is an important part of following Bluebook for scholarship! There is a slim section on how to translate requirements to briefs, but it is underspecified so lawyers just kind of muddle along. All while insisting they follow it to the letter. They don't.