If a bowler takes more than three sequential wickets (say six wickets) in one over, would this be called as two hat-tricks or does this feat, however unlikely, has a different name?
a bowler taking a wicket off each of three consecutive deliveries that he bowls in a single match (whether in the same over or split up in two consecutive overs, or two overs in two different spells, or even spread across two innings of a test match or first-class cricket game).
So a single hat-trick means taking a wicket off each of three consecutive deliveries. So if a bowler takes six wickets in six consecutive deliveries, it will be considered as two [different] hat-tricks. There is no special name for this since no one has achieved this feat but we may say it double hat-trick. However if a bowler takes four wickets in four consecutive deliveries it is called as four in four but the term double hat-trick has also been used in the media*. So there is no special term for that. Everyone refer them in their own term.
* taken from Wikipedia
A 'perfect over' of 6 wickets taken with 6 consecutive ballsbbc was achieved by Australian Aled Carey on 21 January 2017 while bowling for his club Golden Point against East Ballarat. This perfect over consisted of 2 catches, an LBW and 3 bowled.
Wikipedia and his club Golden Point tweet states this as quadruple Hat-Trick, while few media like deccanchronicle, thenews and article.wn states this as 2 hat-tricks in an over and some other media like zeenews, scroll and financialexpress states this as double hat-trick.
So, as hims056 said in his answer, there is no special term for that. Everyone refer to these hat-tricks in their own term.
4 wickets in 4 balls may be called qadri-trick; 5 wickets in 5 balls penta-trick ; Then there is change in the over.Yet, for 6 successes in 6 consecutive events may be called hexa-trick; 7 in 7 hepta-trick; 8 in 8 octa-trick; 9 in 9 nona-trick; 10 in 10 deca-trick; 11 in 11 undeca-trick and 12 in 12 dodeca-trick! How's that!