yes and no
shield is considered a weapon, so any “flat” frostburn on shield will only be applied with shield attacks/attacks that use or scale on shield dmg
but, weapon dmg in regards to true DoTs like frostburn is considered 1 source, even if it’s left/right hand
when we say different sources we mean different attacks; but also different attacks with their own native DoT
ex could be your battlemage with cold Forcewave
Forcewave has its own DoT (trauma) which we can convert to Frostburn
so that will be 1 source of Frostburn
then Forcewave also has %weapon dmg, which will apply global amassed “flat” Frostburn
which then gives Forcewave 2 sources of Frostburn
but since one of those are %WD based frostburn, it means it wont stack with other %WD applied frostburn, highest would tick
suggest you look through this to get a better understanding on true DoTs