In[1]:= Quantity[Quantity[7200, "Seconds"], "Hours"]
Out[1]= Quantity[2, ("Hours")^2] In[2]:= UnitConvert[Quantity[7200, "Seconds"], "Hours"] Out[2]= Quantity[2, "Hours"]
v Mathematica 9.0.1
Well, Quantity[Quantity[7200, "Seconds"], "Hours"] is a bit strange as input. Usually, you'll want to make the first argument dimensionless. Note that value Quantity[unit] is the same as Quantity[value, unit], so your result makes perfect sense.
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