So, I want to do the whole 18 volt mod thing, so I went to my guitar to put the two batteries into my guitar; there isn't enough space in my battery compartment to fit them both. I'm sure this isn't the first time that this has happened to someone, so what am I supposed to do if I want them to fit?

...get a chisel, or if you have no woodworking experience, take it to somebody with a workshop and carve out some more space.

Or, this:

If you're gonna waste time putting 2 batteries in an electric guitar then you might as well just install Seymour Duncan Invaders with only a single volume control or no controls at all.
Cusp of Magic
I have 2 battery's in the controlled cavity on my ltd
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The only real answer to the SG vs Les Paul debate is to get a Flying V and laugh at all the suckers who don't have one.

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if you embrace inaccurate intonation it can be quite arousing.