Yes, you'd need another ball of yarn to work them at the same time. As far as BO/decreases, it's really a matter of preference. I prefer to do decreases on the RS, but they look the same either way. I'd probably go until it's *exactly* the length measurement asked for, and then do my BO on the next row. All up to you!
*Dream Swatch in Madil Eden Bamboo
On the needles:
*Isabella from Spring Knitty
*Striped hoodie from Nursery Knits
*Tomato from Knitting Daily
I'm hunterjenn on Ravelry. Come find me!