Relatively little is known on how Neptune's atmosphere might operate on the scale that tornadoes do. However, the dynamics of Neptune's atmosphere are rather different from those on Earth, with large storms developing from anticyclones rather than cyclones and the lack of a solid surface. Tornadoes as we know them likely do not occur there, but we cannot rule out somewhat similar intense vortices.
If two tornadoes were to merge, they would combine to form a larger and more destructive tornado. This new tornado could have stronger winds and cause more damage than either of the individual tornadoes on their own.
Tornadoes, Hurricanes, Tsunamis, Drought, Earthquakes, and Whirlwinds are all natural disasters the happen on Earth. (Whirlwinds and Tornadoes are basically the same thing)
The winds on Neptune can reach speeds of up to 1,300 miles per hour (2,100 kilometers per hour), making them the strongest in the solar system. These high-velocity winds are due to Neptune's fast rotation and its turbulent atmosphere.
Tornadoes appear to be most common in the eastern and central portions of North Carolina.
A tornado and a hurricane cannot "combine" as they operate on different scales. It is fairly common for tornadoes to produce tornadoes.
Neptune is an ice giant planet and while we do know that it does have storm systems in its atmosphere, we do not know if these are like tornadoes on Earth.
Tornadoes can happen in any place that gets thunderstorms.
No. There are enormous, magnetically-driven vortices of plasma on the sun that have been compared to tornadoes. However, these are not true tornadoes as they operate on entirely different mechanics.
No. Mercury has essentially no atmosphere. There are no storms of any kind there.
Tornadoes happen in Miami for the same reason they happen anywhere else. See the related question for how tornadoes form in general.
The term you are looking for is tornado watch.
Tornadoes can happen in Ottawa. Canada does get tornadoes, though not as often as the U.S. due to its cooler climate.
Tornadoes can happen anywhere in the world except cold areas like Antarctica and the north pole. Tornadoes are mostly common in United States.
There is no evidence of tornadoes on Jupiter. The Great Red Spot has sometimes been compared to a tornado, but in reality its dynamics are completely different.
Yes. Tornadoes, including some significant ones, can and do happen the the Philippines.
Tornadoes can happen anywhere - they're usually formed from severe thunderstorms.
Most tornadoes in the U.S. happen in spring and early summer.