Dear Mathew,

1)Both pressure and temperature are considered to be present simultanously only difference they are treated separatelly because they falls in two different catogory of loads.i.e.primary and secondory loads.

2)Primary loads are those with which you make equations of equilibrium for solving unknown.For secondory loads equilibrium law is not applicable because stress due to secondory loading(secondory stress)is self limiting

3)The stress due to pressure loading is considered as primary stress while the stress due to temperature loading is considered as secondary stress.

4)Code has develoved seperate equations for calculating and limiting sustained(pressure effect)and displacement stress range(secondary stress-temperature effect).

Hope, I answred to the satisfaction.

Regards,
Aslam