Both are widely considered the most successful teams domestically with Juventus winning 27 or 29 scudetti (depending your view point) in peninsula and United bagging 18 league titles (given the 18th is almost bagged) and 11 FA cups in England. They've been often painted as a lucky team scoring many late goals over the years. Also the number of contentious calls that seems to go their way when they're in dire need of help and are said to benefit more than other clubs from officials.
Their fans are often termed as 'gloryhunters’ and not local – i.e. they only support the club because they are successful. This can be observed, apparently, by the relatively small number of followers from Turin and Manchester with most Mancunians are City fans while Turin-locals supporting Torino therefore the two titans get the majority of their support from the southern part of their countries.
Number of coaches carrying Manchester United fans on motorways leading to Greater Manchester when United are playing at home and 100s of Juventus fan clubs all across Italy organizing trips to Turin and away games (which is by the way a very profitable business) is a testament to that. Also the tremendous support these teams enjoy whenever they travel down south to play domestically.