why do you need to merge them? You can downlaod your ancestry tree using GEDCOM and then upload it into familytreemaker or similar and so possiblt you could do the same with heritage and then go through your familytreemaker merging the people and then upload the final result back to either/both ancestry or heritage
I need to merge them basically because of ill discipline, Ill sometimes add people to one but not the other, it's got to a stage now where there's 100s more on ancestry than heritage.