So I'm not sure if this should be in the rules, or the advice forum, so my apologies if I'm in the wrong area.
I want to design a weapon based on the Butterfly sword concept of two swords nested together drawn at the same time and separated as a free action into the two swords. So why not use a set of butterfly swords? I want to have the swords be "bigger" and deal more damage, and also I can't find anything on "Not separating the swords, and swinging with both at once".
The idea would be a two handed sword, with the ability to split into twin swords. Is there a weapon similar to this in Pathfinder anywhere, or rules to make it using the custom weapon rules? Or has anyone built a weapon like this in the past?
There's a guide on how to create new weapons in pathfinder. I believe it's in the PPC: Weapon Masters Handbook.
I'd be very mindful of that subsystem, since the design points are not very well balanced. You'll often end up with weapons that are objectively overpowered or underpowered depending on which weapon properties you pick. You're better off just comparing to existing weapons and adjusting accordingly, in my opinion.
As a GM, I'd be tempted to say, "It's just a double weapon. One weapon, so one draw action. Two ends, so can attack with both. Enchant/Enspell separately. Will need a specific proficiency..."
At which point, the rulesmaster at the table will chime in, "If you need a proficiency from a feat, just take Quick Draw instead."
Then we'd all smile, and agree it's probably NOT a specific proficiency, but it's almost surely an expensive or masterwork item.