Answer:
Two in a whole and four in a pair. And six in a trio you see. And eight's a quartet but what you must get. Is the name that fits just one of me?
What do you use to hoe a row, slay a foe, and wring with woe?
I am the fountain from which no one can drink. For many I am considered a necessary link. Like gold to all I am sought for, but my continued death brings wealth for all to want more.
My second is performed by my first, and it is thought a thief by the marks of my whole might be caught.
Looks like water, but it's heat. Sits on sand, lays on concrete. A play on the eyes, but it's all lies.
Keep doing me to avoid lens dryness