Following up, I eventually DID find a very compact twist locker by Mammut, this 24kN rated, 4" high little "Micro". Still have yet to try it but I believe it will sub in for the small blue fusion just fine, keeping the ropes staggered. Now when I'm working or rec climbing I have an added margin of safety, and even after 15 years that never gets old! $12 incl. shipping from Amazon, that's a darn reasonable deal.
Funny how when I go online shopping for a particular size in PPE, there's not always info on the dimensions of the piece. Strength rating sure, but it's like pulling teeth to find length/width measurements on too many sites. SIZE MATTERS when it comes to interacting with other gear. Ha!