Well, even though it's not supported by the rules, I went ahead and said you can have Double Flaming, with both flamings activated by the same action. Primarily I did this because I just never liked how it was ALWAYS only 1d6 damage, it's just too inflexible.
Yes, it makes Burst weapons less powerful (except that as part of my Material rules, certain metals/crystals reduce costs of certain abilities, so for some weapons Flaming Burst only costs +1), but that wasn't the big problem I was encountering.
I was seeing people say "Okay, I want a Flaming Frost Shock Sonic Longsword". Frankly I'd rather see "Longsword of Really Hot Fire" than that sort of mixing.
As a side effect, it also sorta killed Bane weapons. Why bother with a Trollbane weapon when you can get double flaming? Bane was a bit weak to begin with, too.
There's one thing people haven't considered when comparing Double Flaming to Flaming Burst: elemental resistances/hardness. If the target has Fire 10, a double flaming sword only gets past that 8% of the time, and only for 1-2 damage. Fire 15 stops it cold (no pun intended). Too many times when fighting outsiders or such, you end up writing off the ability entirely.
On the other hand, a Flaming Burst weapon can't get past either on a normal hit, but a x2 weapon on a critical gets past Fire 10 about half the time, doing up to 6 damage, and can even get past Fire 15 if you roll perfectly. If it's a x3 or x4 weapon, you'll practically always get past it, assuming you crit.
(Keen Scimitar with Improved Critical = lots of crits)
If you don't like letting the Flamings stack, just make some new weapon enchantments:
GREATER FLAMING: +2d8 fire damage, sets objects on fire (see special rules for that), acts as a light source, Market Price +3, requires Delayed Blast Fireball, doesn't stack with Flaming.
and so on.
Or, one that I like, that came from this board:
SEARING: When used on a weapon with an elemental damage enchantment (Flaming, etc.), increase the elemental damage die one size (so Flaming does 1d8, and burst abilities do 1d12). Also, the weapon bypasses Elemental Resistances (but not immunities or object hardness). Market Price +1.
(It's like Sure Striking for elemental damage, in a way)