Right now there isn't a built-in way to merge two date/time fields into one field.
However, with custom code and our API, you could do this.
Zapier or another third-party tool could be used to grab the values from the two date/time fields and pass them into the 'merge field' via our API when a form is submitted.
We don't currently have any examples for doing this in our API documentation, though you can reach out to the community for help!