I doubt it. The SGB-30 has a glue-on neck, whilst the SGD-612 has bolt-on necks. I'm not saying It can't be made to work, but you'd have to do a lot of work with the neck pocket to make a glue-on neck fit. You'd probably have to cut away the bridge of wood between the neck pickup rout and the neck pocket so that you could fit the neck tenon in, which will probably also mean routing out the neck pocket and neck pickup rout to be deeper.

If you can do all that then you're not all that far off being able to make your own body for it!

Worth dropping Adam a line to see if one of the factories can supply an SG style bass/6 string kit. If not, could they do a custom kit routed for an SGB-30 neck and an SG or ES-1 neck with the basic body but routed for your bass pickup choice (there are very few humbucker sized 4-pole pickups) and I'd suggest no holes for bass bridge, just a ground wire hole and use a P/J-bass
style bridge instead (as the kit Gibson style one is pretty useless).

The factory can only say no, but one of them may already be making a similar double neck for someone else.