My mice are up on a tree and a big stream of water is under them because a dam broke.
So I thought: Hey, let’s make a float and navigate the stream.
For me that is two obstacles: First: craft a float. Second: ride the float over the stream.
1st one is Boatcrafter. Then I looked for a skill to test crossing the stream and it looks like it’s Boatcrafter again (“he can also use his knowledge to navigate waterways” p. 244). Pathfinder doesn’t really fit imho.
Can you test the same skill twice? Doesn’t that conflict with the rule “Fun Once! Let’s Not Do It Again!” (p 90)?
Or would you say it’s 1 Boatcrafter skill test but with a higher difficulty?
What is it about crossing the stream that introduces the possibility of failure? If there isn’t any reason why they can’t just ride across, I’d just let 'em ride across (after making a boat of course).
In situations like this, I try to identify where the most interesting failure lies, and use that test as the crux. Piloting the boat has much more potential than crafting the boat, so I would make the test boatcrafter to navigate the stream, but still allow help from skills like carpenter and scientist seeing as the fictional situation requires the patrol to construct a boat.
It’s one test, because they have one goal (“We want to get across this river.”) and one method that they are using to do that (“We are going to make a boat and take it to the other side.”). Any kind of benefit to one part of that task or the other would be treated as help or equipment, as it would aid the whole process, as either better construction or better piloting/rowing would ease the crossing. The difficulty would be the highest obstacle (here, I assume, it is a seasonal obstacle?). Note that failure could just as easily be a flaw in the construction (“It’s breaking apart!”) or the piloting (“I’ve lost control!”) or a compounding of both (“I am having enough problems handling this thing without it taking on water!”). The interesting part is the resolution, whether it is a twist (“You manage to keep the boat afloat, but are not able to land until far downstream from where you intended, and now have a significant journey [a Journey Conflict] ahead of you.” or “You land the boat, but not where you wanted to. You were planning on sneaking up on the cave and rescuing the trapped villagers, but now you have no choice but to cut through the open meadow beneath the hawk’s nest.”) or simply assigning conditions (“You make it, but you all had to paddle with all of your might, and that didn’t stop the water from coming over the side of the boat. You are wet and weak. Everyone mark Tired and Sick.”).
Note that if you really think that crossing the river is a BIG DEAL then you could make a variant Journey Conflict for river journeys, but that suggests to me something more like a long trip down rapids and whatnot, rather than a single crossing. YMMV.