https://open.mapquestapi.com/staticmap/v5/map?key=KEY&locations=Denver,CO||Boulder,CO&defaultMarker=none&size=1100,500@2x
This can also be done on individual locations using an option, as seen below.
https://open.mapquestapi.com/staticmap/v5/map?key=KEY&locations=Denver,CO||Boulder,CO|none&size=1100,500@2x