Hi there,
I'm not sure why your Span checkbox is not available .. Might be a form level template setting.
Are these two fields editable or display? Might make a difference in how you approach this.
BUT, if it were me doing what you want, here's what I'd do:
1. Add both the fields to the form in said order.
2. "Code" field has the prompt text.
3. "Code" field is set to NOT be the last on the row, and to NOT be the last in the cell.
4. Span the "Description" field, making sure it has a blank prompt and no comment.