Very simply, how would these stack?
A soul forger adds his magus class level on Craft checks to manufacture armor, shields, and weapons. This bonus applies on skill checks required when using Craft Magic Arms and Armor.
At 7th level, a soul forger uses the 1/10 gp value of armor, shields, and weapons to determine how much time it takes to craft mundane items, and he requires only half the normal amount of time to enchant magical arms and armor.
This ability replaces knowledge pool.
At 5th level, a forgemaster can craft mundane metal items quickly, using half their gp value to determine progress, and can craft magical metal items in half the normal amount of time.
Non-conflicting parts:
* Adding the soul forger's magus level to craft checks for mundane armor/shields/weapons and to skill checks for magic arms and armor will work without question.
* The soul forger's "1/10 gp value for craft times" only applies to armor/shields/weapons while the forgemaster's "1/2 gp value for craft times" applies to any mundane metal item.
Conflicting parts:
* If crafting a metal weapon/armor/shield, do you take the better of the two bonuses to determine craft time? Or do the bonuses stack? If they stack, is it sequential? Multiplicative? (I'd lean towards only the better bonus working)
--> A forgemaster uses 1/2 an item's gp value to determine progress, while a soul forger uses 1/10 the item's value. So the better bonus is the 1/10 value.
* If crafting a metal magic item using Craft Magic Arms and Armor, is the enchant time halved (both archetypes offer the same bonus), or do both bonuses apply and make the craft time 1/4 the normal time?
Follow-up question:
* Does the forgemaster's time reduction in crafting apply to any magic item made of metal or just "Craft Magic Arms and Armor"? As it is written now, it would apply to any metal Wondrous Item, Rod, structure (time to build that magical wall made of iron!), etc.
As I, too would like to know the answer, I do some threadnecro.
I would let them stack down to 1/4th on the strength of:
A- These are bonuses from different sources
B- It is a highly specialized character, so why not?
If you have to burn 12 levels of character development to get to this question in the first place... is it really unbalancing?
RAW wise...
Both class features are named "Master Smith" and there is no language in the feat saying "if you receive this ability from another class here is how they stack" so they probably do not.
Ultimately this is a GM call, since neither class is playable in Pathfinder Society as written, an 'official' ruling is somewhat moot.