The ignite command for the Solid Rocket Boosters (SRB) would not be issued unless all three Space Shuttle Main Engines (SSME) were at least 90% of thrust. The launch would have been aborted if one or more of the SSMEs failed to reach required performance before T-minus zero. Abort was possible because the SSMEs could be shutdown after starting.
The SRBs, of course, couldn't be shutdown after ignition. Once the SRB ignition command was issued, abort was impossible (unless maybe both SRBs failed to ignite?).
If only one ignited, what would have happened?