We have a Blacksmith and a Bowyer in our group that wants to attempt to combine their skills to try and create a crossbow. How would I go about them doing so? Would the higher-skilled Blacksmith get to add a bonus or would it be the Bowyer that had the Blacksmiths help and they would make the roll with a bonus?
Okay, so here's how it goes down.
Chances are that the bowyer can't even apply their skills since Craft: Bows isn't used for Crossbows. So who has the highest Craft: Weapons skill bonus?
The one with the lower bonus can attempt a DC 10 Aid Another action. If they succeed, the one with the better check who's making the weapon will get a +2 to their roll.
The Craft DC for any Crossbow is 15, but you can choose to accelerate the crafting by applying a +10 to the Craft DC (since the crafting process is the multiple between the Craft DC and the Skill result). Since it's also MWK you'd make a separate Craft roll for the Masterwork part that has a Craft DC of 20.
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Those are the rules, but the crafting rules are pretty awful. I'd let the Bowyer make the string and carve the wooden body while the blacksmith creates the mechanism. Then they can both roll their two craft skills, add their crafting process together, and feel like they're both contributing.
Having to spend 8 hours each day for a month or longer just to provide a +2 bonus is demeaning.
Yeah...
If you want to make it interesting, it's gonna take some houseruling (and I'd be careful about that).
The unchained crafting should make the process a tad faster, but I still have a hard time imagining it being worth it. Saving a couple hundred gps versus days - if not weeks - of work is a hard sell past the very first levels.
Beyond that, aiding another is not all that friendly. It's not useless at low level, but a +2 is what you'll get, as always - and hirelings can do that for relatively cheap without immobilizing a PC.
Plus, as pointed out, neither of your PCs have the skill needed for crossbow making as is, strictly speaking.
I'd say it depends on what both the PCs ad you the GM want out of this.
If they're in it for the fantasy, and you want to encourage them and make it fun for them, it might be better to make stuff up. Just be careful it doesn't get out of hand.
If they just want to save money, point out that what little they save will be irrelevant really fast. Then maybe look at the unchained crafting, so that it doesn't ground them for months at a time.
Thanks for the answers folks, its much appreciated!