I am doing a static Fatigue analysis and I remember from the past posts in this forum that CII would double the allowables extracted from the available curves since it assumed the peak to peak range of the cycles.
in my case, the min and max of the pressure/temperature fluctuation curves are higher than the ambient condition ( Temp. from 15 to 75C Press from 2 to 26 barg - ambient temp. 4C)
I would like to know whether or not I still have to use the "doubled" stress allowables.

thanks in advance

What fatigue (S-N) curve are you using? Is S labeled stress amplitude or stress range?
In CAESAR II we typically run an expansion stress range calculation. You can throw pressure in there as well.
Amplitude is zero-to-peak, range is peak-to-peak. So if you are evaluating range, use twice the amplitude value.
You seem to question the limits of your range - whether they are both on the same side of "ambient" or on different sides of ambient. Those limits define the boundaries of the range calculation and it doesn't really matter if they cross ambient conditions as long as you evaluate the complete range.
For years, CAESAR II recommended range calculations between each operating state and the ambient (or installation) condition and it was up to the designer to build the range calculation between operating states if one of those states is on the "other side" of ambient. The current version of CAESAR II (by default) now recommends all ranges between operating cases as well.