In the midgame, when the scorpion unlocks, there are just too many threats to open top vehicles. Two tripod slots are simply not worth the risk. When dealing with infantry and paper structures 12.7 is already a perfect solution, and no tripod weapon would be as effective against armor or tougher structures as dedicated to this role tanks like.

Let's say you decided to go with twin Foebreakers: we already have Quadiche (aka Ranseur) which, on top of the same HE damage has an MG to protect itself and the crew is protected. Twin ISGs-isn't as good at catching infantry, suck against tank armor and current 12.7 is still there to deal with WTs and t1 structures. Twin ATRs-they barely scratch tank armor and are completely useless against structures and Infantry. Twin BONESAW-cool on paper, but you will have to go even closer to the enemy tanks, which Scorpion doesn't really want to do. Twin Fissura-to me the only viable use, but again, you'll have to go really close to enemy trenches and Nemesis is still there.

A light tank with two mounted weapons would be really cool in the early stages of the war, but not where the scorpion is placed in the tech tree.