Does Steady Sniper gear boost (Starfinder Armory p.154) allow unwieldy weapons to make multiple attacks? If not, how is someone supposed to use it with sniper rifles, which are almost exclusively unwieldy?
It is an exception. It does nothing for unwieldly, but specifically allows a sniper weapon to shoot twice.
Specific rules trump general rules. The specifics of two sniper attacks as a standard, in this case, trumps the general unwieldly quality. However, only on weapons that have the sniper quality.
Maybe I'm wrong though. Maybe it only works with the Coil Rifle, the only sniper weapon without unwieldly.
As an additional note, Steasy Sniper also has a number of caviats for its attacks.