Bases together would get you around 3.5~ inches, at least with my Reolleicord here.

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Of course, since you're shooting square, they don't need to be both facing up. If you can get a little more involved with the mounting, you can put two TLRs side-by-side, wherein one is upside down and get an even tighter spacing… with my Rolleicords, it's right around 3".



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