I did some math and have figured out that I can do the first border in skein 3 and the second border in skein 4.
My first Watership Down was done in Selene, which has 875 yds/100 grams. This shawl is being knit in Eos, which has 1260 yds/114 grams.
On my first Watership Down I used about 24 grams for the first border. To make the math even easier, let's call it 25 grams. So, dividing 875 yds by 4 gets me 218.75 yds, which I'll round up to 220 yds. Doing the math (220 yds)*(114 g/1260 yds) is approximately 20 grams.
Similarly, I used 28 grams on the second border, so the number of yards I used would be (28 g)*(875 yds/100 g) = 245 yds. I rounded that up to 250 yds and calculated the amount of yarn I would need as (250 yds)*(114 g/1260 yds) or about 23 grams.
Pattern: Watership Down by Janine le Cras
Yarn: The Unique Sheep Eos in Moulin Huet
Needle: US 3 (3.25 mm)