In case you weren't thinking about it, the size of the knobs for bass tuners is usually your limiting factor. I'm not sure how close together is acceptable, but for your purposes I'd pick smaller knobs. The Hipshot ultralights have different options, the dimensions page shows the sizes of the knobs. With the smaller ones, it looks like you could do spacing of about 1.4"-1.5".
EDIT: Link to the ultralights dimensions document.