I might temper Nigel's advice about replacing an over-dissipating resistor or resistor pair with a higher wattage assembly. Assuming the circuit was designed properly and worked fine originally, an overdissipating resistor is usually an indicator of another problem in the circuit causing excess current to be drawn. Replacing the part(s) with a higher-wattage would be similar to replacing a 3 amp fuse with a 10 fuse to keep it from blowing so often.
Another reason for using two 10's in parallel vs. a single 5 is that "5" is not a standard value. In some circuits, 5.1 is not close enough or 5.1 isn't available in that high of a power rating and the 4.7 that is available is too far off the design value. Two tens in parallel gives the required 5. That may not be the reason here ... it just answers the general question of "is that another reason to use parallel resistors".
I've often seen parallel resistors, as many as four, used in the emitter circuits of power amps. Finding resistors of low values can be a problem that paralleling answers. Also, the lower the value, the more the solder connection and leads contribute to the overall resistance. It may be that several larger resistors in parallel will take care of that problem.
A careful calculation may find that two resistors in parallel have more surface area than that of a single resistor of double the power rating. Or two resistors give better air flow for cooling.
When power resistors are replaced, you should mount them on a PCB with a bit of clearance, 1/8" is usually good, for good air circulation and to keep hot resistors from browning the PCB.
Dean