That's a great question.
The error in indicated altitude arises when the air temperature at a particular altitude differs from the standard ISA model. This temperature difference affects the density and therefore the pressure of the air
The mechanism in the altimeter is constructed assuming a particular rate of change of pressure with altitude. Warmer-than-standard conditions will cause under-reading and colder-than-standard will cause over-reading, which is the dangerous case. This has been well explained in previous posts.
The important point raised by your question is that 'aerodrome QNH' is deliberately adjusted for any variation from standard temperature so that the altimeter will always read correctly when at the aerodrome surface. The error in indicated altitude will only become apparent when the aircraft is above the 'QNH reporting station'. This reporting station is the one providing the QNH, in other words, the aerodrome. The error will be small at low altitudes above the reporting station but can be significant at higher altitudes.
The magnitude of the error depends on the average temperature deviation from ISA of the column of air below the aircraft, or to be precise, the column of air between the aircraft and the reporting station. The error approximates to 4 feet per 1000 ft above the reporting station for every degree C deviation from ISA.
So, to get to your question, the error will be the same as both aircraft are 5000ft above their respective reporting stations. This assumes that the two columns of air have the same average deviation from ISA.
On the other hand, an interesting comparison could be made between two aircraft, the first flying at an indicated 10,000 above an airfield situated at sea-level and the second flying at an indicated 10,000ft above an airfield with an elevation of 5,000ft.
Assuming that both are using the QNH of their respective airfields, and that both are flying in an atmosphere with an identical colder-than-standard deviation from ISA, the aircraft will be at different true altitudes as the height above the QNH reporting station for the first aircraft is twice that for the second aircraft.
Assuming an average temperature deviation of -10C for the air beneath both aircraft, the first will have an error of 4x10x10=400ft whereas the second will have an error of 4x10x5=200ft.
Last edited by eckhard; 18th Jan 2017 at 22:23.