Most airlines in the States do not use tag-on scheduling anymore. Maybe
Southwest comes closest to doing this on their route structure. UA used to have a IAD-PHX-TUS some years ago using an Airbus. The tag on is consolidating routes but IAD-PHX probably generates much more traffic then
IAD-TUS. If you want to go IAD-TUS you would have to connect at ORD
or DEN.