I've got a pair of EMG's in an RG7321 but there's not enough room for a switch, barely enough for a battery. Can I possibly hook up two volume knobs without a tone knob or a switch? I've tried finding diagrams of something like that but unfortunately I can't find one
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you can, this way you'll be able to blend the pickups in and get that middle position, however you can go with a single volume and a DPDT (on/on/on) mini toggle switch instead for the 2nd position, the size is similar to a volume pot and the shaft clearance is similar.

question is what kind of emg? active or passive?
Active, just the 81-7 and 707
Baby Joel's rabbit profile picture is kinda cute. #TeamJOEL
now there's two ways I see the hot lead escaping from the guitar but right now in theory this seems more practical , it's inspired by a single volume no tone wiring. But anyone else who reads this and knows wiring feel free to jump in. It's active so it shouldn't need a string ground