I transited IAD a couple of days ago doing LHR-IAD-IAH. Immigration wait in midfield terminal was about 45 minutes, then a short bus ride to a G Gate for the UA Express flight to IAH. G gates are simply exits off a long walkway system attached to what appears to be a converted cargo shed beyond the midfield terminal; it's rather crude but appears to work! On this basis a 2 hour transit would seem to be do-able.