Can I create a workflow that is the "second step" of another workflow?

This can be done, but probably not in the way you’re thinking of as a trigger. 

Since goals are based on segments are events, you would just use those segments as the triggers for the following workflow. 

If users can enter that goal segment in other ways than just entering the campaign, then you just need to add a couple of extra creative filters to make sure you only include users who entered the campaign. You could mark contacts with an attribute, use a segment querying if they were sent an email from the campaign, etc. 

Yes, You can set the second campaign as a manual segment trigger and add the manual update to the first one. This way you can validate for instance, if they opened an email from the first campaign or use a “wait until” they do some action or reach a certain data point. 

For metrics and sanity’s sake, I would recommend using “wait until” with a max time. If they fail out of the wait, you can add them to a manual segment of users who never opened your first campaign within the timeframe for instance. 

If you want a way to leave them in step one UNTIL they perform an action, you’ll want to use the wait until and have it check a data point that gets updated.