That’s a great question, not really one that I’ve put thought into yet myself! There doesn’t seem to be a built-in way to sort by multiple fields at the moment, so maybe it would be worth a feature request to see whether it’s possible.
After giving it some thought, maybe a way to achieve this could involve a nested loop where the outside loop is basically just grabbing all the date fields from posts and then the inside loop is re-grabbing all the event posts that have the same date field and sorting by the time field. The issue with that though would be that the inner loop would run once for every post, so if multiple posts share the same date, the inner loop would display the same group of same-day events as many times as there are events in that group. You could probably use some conditional logic to get around that.
I haven’t tested this to see if it actually works so you’ll probably want to test and troubleshoot this, but here’s sorta what I’m thinking:
<Loop type=events orderby_field=_StartEventDate>
<If check="{_StartEventDate}" is_not value="{Get last_event_date}">
<Loop type=events custom_field=_StartEventDate custom_field_value="{Field _StartEventDate}" orderby_field=_StartEventTime>
<h2><Field title /></h2>
<div><Field acf_date=_StartEventDate /></div>
<p><Field acf_time=_StartEventTime format="g:i a"/> - <Field acf_time=_EndEventTime format="g:i a"/></p>
<If count more_than value=1><Set last_event_date><Field _StartEventDate /></Set></If>
</Loop>
</If>
</Loop>
This markup has two loops, two conditional logic statements, and a variable that gets set just about every loop so this seems super inefficient and could almost definitely be improved with some rewriting. Someone with more experience could probably suggest something better, but I figured this might give you a way to get started in thinking about this. If the markup didn’t have to run the inner loop for events that were on a unique date, that might improve things, but it would probably involve using the Template tag to avoid writing your display markup (the h2s and divs) multiple times.
Hope this is helpful!