Typically studs are spaced 16" on center but that's a general rule that can change. It's hard to find studs behind tile as you're figured out. Assuming 16" OC you can angle the bar to hit both studs. You can use toggle bolts and they'll probably be strong enough. You need to apply some back pressure as you tighten the screw. In the case of several in the same bracket, you need to apply some pressure and tighten each screw a little at a time. They also make special holders for grab bars. Off hand I can't remember the name but their quite heavy duty and unfortunately rather expensive (although far less expensive than a fall and trip to the hospital).

here is my link to my reveal several years ago. http://ths.gardenweb.com/discussions/3333818/my-west-bathroom-reveal-from-project-in-2013-to-2014?n=15 apparently the link in the tread to show my pics doesn't work any longer :/ I'll see what I can do to that. here is the floor with the shower pan. The Hydroban wall moisture barrier is not showing any longer, its under the shower tiles: Read the reveal text that I linked above. It will tell all about everything I used in this bath remodel: Floor with unfinished vanity, across from the shower. There are 2 doors into this room, the one you see here and on the other side of the room facing this door. Its my bathroom hallway, lol: Have to show my laundry center, across from the toilet. My beloved Miele washer and dryer:

I'll happily install them. I would rather do it myself instead of having the GC's guy/gal screw up my hard work then call me back to fix it. The tile setter has the proper diamond bits / hole saws and understands the waterproofing system used. The GC will pull out the hammer drill & masonry bit. Choose wisely. https://www.ceramictilefoundation.org/homeowners-guide-to-hiring-qualified-tile-installer

Ok, that makes it tougher, but one of the better electronic stud finders might still locate the studs through the tile. I prefer to mount something that needs a lot of support - like a grab bar - only in studs. If you can locate the studs, but can't find the right length grab bar to perfectly match the stud spacing, one solution is to buy one that is slightly longer than the stud spacing and mount it at an angle that allows the ends of the grab bar and the studs to align.