I’ve always liked flip-top toothpaste tube caps, which Colgate tubes have and Crest tubes don’t. I’m sure there’s a reason why they use different threads; perhaps there’s a standard for toothpaste tube threads that encompasses both?
Anyhow, after years of pondering this dilemma, I jammed a Colgate cap and the top of a Crest tube onto a length of 5/16″ drill rod and eased some epoxy into the joint:
It turns out that the minor diameter of the Colgate cap is just slightly smaller than the major diameter of the Crest tube, so they don’t quite slide together. The epoxy makes for a perfect, zero-clearance fit that’s so tight you must crunch the tube to unscrew it:
For what it’s worth, that buttress thread form provides a leakproof seal in the original tube.
I have no idea whether this will actually work, because the closet has a three-pack of Colgate that should last for quite a while. Yes, we tend to buy whatever toothpaste seems cheapest on a per unit basis when we’re restocking the closet…
The Smell of Molten Projects in the Morning 
From the RSS preview I was expecting this to be another ToM project. But even so, you, sir, might possibly have too much time on your hands. :)
Well, there I was in the Basement Laboratory Meditation Annex… with a bit of time to spare… [grin]