What I've done in the past is use conditional formatting to change a column when a specific event happens. So, the most common way this is done is when a certain field equals a certain value, it will automatically flip another field to a specific value you set, hide the field, or show a different feature thats not available unless that specific first field equals a certain value...
Maybe this might help you? (not affiliated with the business below)
inclusivedesignsolutions.com link
Here's another suggestion @jessica seeing your most recent comment.
You could have a status field with the status items of your choice and set workflow steps dependent on the choice within that field:
New Workflow
UPON CREATING NEW ITEM:
If 'Current Item:Status' equals created
> then email 'Current Item:CreatedBy'
(include email that notifies creator of item created?)
> then email 'Current Item:ModifiedBy'
(email instructs the user to go into the form and flip the status field to processing)
<<<>>><<<>>>
New Workflow
UPON ITEM CHANGE:
If 'Current Item:Status' equals processing
> then email 'Current Item:CreatedBy'
(include email content here that you would show the creator at this time)
> then email 'Current Item:ModifiedBy'
(include email notifying them of item change)