(03 Aug 2015, 8:35 pm)Mark P wrote Pure speculation on my part but how about:
X66 via A66
X67 via A67
Apparently the X67 will go via Longnewton so here is my wacky suggestion.
X66: current route.
X67: via, Freeman's Place, Haughton Road, Haughton Green, Stockton Road, Great Burdon (A1150), A66, LONGNEWTON Darlington Road, ELTON, then A66 to Stockton and Middlesbrough the same route as the X66.
Pure random speculation this. Both would be half hourly (combined 15 minutes).