What addresses are on the marriage certificate, and how far are these places from Calvary? People were often prepared to walk/travel to the Chapel of their chosen denomination, often passing several other plaecs of worship on the way.
I'm not that familiar with the area but I know that Treforest is named as a place within the boundaries of Llantwit Fardre, as are Tonteg, Glyntaff, etc. See the Genuki page on Llantwit Fardre. There were several pubs called "Colliers Arms", as you would guess! On Slaters 1858-9, Elias Thomas was at the Colliers, Treforest.