You can show multiple roles/designations for the same company in the following ways on Resumonk.
Add two experience entries, but enter the company name only in the first one.
You can then list your achievements in the respective 'Details' fields of these entries.
Or you can use just one experience entry for that company and list both the positions in the 'Details' field. You can either leave the designation as blank or write 'Multiple Positions' in it.