Fire+Air grenadier viable?
They share the same CD, so no.
twitch.tv/aliettefaye
All sigils share the same CD. This is true whether the CD triggers on swap or on crit, or on some other condition. As such I generally recommend you only take one sigil with a CD, and one sigil with a constant passive effect.
Also, even if Fire and AIr did proc at the same time, it’d only be, like, 1.2k tops, so it’s not really worth it considering you could just have taken two Sigils of Force or Accuracy or something.