This really isn't the right forum for the question, but I'm going to try and answer it anyway.

The simple answer is: Hurricanes don't merge, as such.

Lets look at the physics of this:

1. Themodynamics: Two engines drawing from the same fuel source means that two engines will run out of fuel faster than one engine. A hurricane is an engine that runs on warm ocean waters, meaning that eventually both storms cannibalize the ability of the other storm to survive and/or strengthen.

2. Conservation of Angular Momentum: Two gears spinning the same direction (lets say clockwise for arguments sake) smack into each other. If the gears are equal size, they will both stop spinning. If one gear is smaller than the other, the larger gear will slow while the smaller gear will stop spinning. You can think of a hurricane as a giant spinning gear. When two storms near each other, the shear from the larger one will litterally rip apart the smaller storm, while the larger storm will be weakened by the shear from the smaller storm. The closer the storms are in strength, the weaker the resulting storm will be.

So what will happen if Fiona and Earl continue to be so close together? Eventually Fiona will probably fall apart and then Earl will stop feeling the negative effects of Fiona. This will in turn allow Earl will strengthen.

Many times this is described as "absorbing" or "merging" but in reality is is a result of the negative effects of the weaker storm no longer being present.

You also want to look up the Fujiwhara effect which affects the motion of storms in close proximity.