So, buff stacking hasn't changed much on live since the classic era, which means that
for the most part we can rely on live spell stacking data to determine stacking here (there's always the possibility that stacking changed, and if so you'd need to talk to a classic researcher).
https://lucy.allakhazam.com/ is the king when it comes to that kind of data, and we can see that on live EQ ...
The Aura of White Petals (
https://lucy.allakhazam.com/stacking...76&source=Live) and Circle of Summer (
https://lucy.allakhazam.com/stacking...52&source=Live)
should stack: neither is listed on each other's "Stacking" list, and those lists indicate the Aura is slot 2 while the Circle is slot 4.
This is all (correctly) reflected on our wiki's buff lines page (which I made largely based on the Lucy data):
https://wiki.project1999.com/Buff_Lines#Cold_Resistance