Is it legal to double lug a fused cut off switch?

If I wanted to add a sub panel, would it be legal to just feed it off the fused cut off switch that feeds the main panel? I'm guessing no, but just checking. I may be digging my foundation and while I'm there I will run the conduit for my garage sub panel and run the appropriate wiring. Physically it will be easier to feed off the cut off switch, because of where I'd run the conduit, but if it's not legal then I will need to run it a slightly different way. The fuse is rated at 100 amps which is the service I have.