Is it safe to mount fender stays and disc brakes on the same bolt? I bought these fenders recently:



The rear fender works fine — there are enough mount points to support my brake, kickstand, rack, and fender.



But when the salesperson sold me the fenders, we both failed to realize that my bike's front brake uses both of the mounting holes available on the left side of the fork:



I went back to the bike shop, talked to a tech and he gave me a longer bolt, recommending that I just mount the fender stay on the outside of the front brake (with some washers surrounding it). But when I asked at another bike shop, someone told me to definitely not try mounting them on the same bolt.

Could mounting them together cause any problems with brake alignment or reliability, or is it fairly safe?
Do not do that. That's dangerous because those stays are often made of soft steel and it's easy to bend or damage them, which means they'll pull out from under the bolt head. In that case you end up with a loose bolt on your brake caliper. Which is bad.

Two alternatives spring to mind: fenders that mount directly to the fork via clips or bands (or hose clamps if you're DIY), or the short fenders that only attach to the fork crown (I think the latter are decorative rather than functional, but enough people disagree that you can still buy them). I would just use cable ties to attach the fenders to the fork and see what happens. I think that will work, but if it doesn't I'd use hose clamps instead. Careful not to overtighten the clamps - you can crush the fork tube which will damage the suspension.

If you're committed to attaching to the brake the safer way is to make a plate that links both bolts and has a tab to bolt the mudguard onto. That way the plate can't rotate and is fairly solid so it's not likely to shift or deform and leave to with a loose front brake mount. The failure mode will be that extra tab bending or breaking, and with a little thought to the design that will be annoying rather than dangerous.