You do not benefit from Song of Flame if you are in Radiance. The advantage here is that you cast Radiance one after another to alternate between providing Song of Flame or benefitting from it. The person using Radiance generates orbs of light for the other sunsinger(s) while providing them with Song of Flame, then they do the same with theirs once the first Radiance ends. Six Sunsingers with Song of Flame and Radiant Will can keep the effect going for two full minutes from their initial super charge if cast one after another, and repeat the cycle multiple times depending on how many orbs are generated in the process. Alternatively three or four Defenders with Crest of Alpha Lupi and two to three Sunsingers with Praxic Fire and Song of Flame could effectively have endless infinite grenades for the entire team through orb of light chaining.

Things like this usually don't stack, but I've never seen this tested so it's possible I suppose.