Order entries by custom date and time fields
You'll want to change that startTime to field_startTime.

Anything in the orderBy gets passed directly onto SQL and Craft is smart enough to do the replacement for startDate behind the scenes, but columns in methods (like time()) or aggregate functions need to be manually prefixed.  This should work:

{% set allEntries = craft.entries.section('sparkSchedule').limit(null).orderBy('startDate asc, time(field_startTime)').all() %}