How you combine them depends on the application, but taking the mean is a reasonable approach. If you have one measurement which is X~N(100,1) and another measurement that is Y~N(110,5) then the mean will be approximately N(105,2.5). This is exactly what is expected with the propagation of errors formula.ok... but still, how do I combine them into one result.
Perhaps you mean that your measurements would be X~U(99.5,100.5) and Y~U(107.5,102.5) and you are objecting to using the normal distribution for these? I am not certain if the propagation of errors formula depends on the specific distribution.