note that everything old world is farmable with a single paly, but somethings just take loooong. Your paly tank is literally invincible, but has typically lowish dps.
Adding a DPS toon to it would speed things up and make themmuch easier i reckon.

For example just think Thekal in ZG for the tiger mount, you need to make sure they die together, and one of them heals. if you kill too slow then you might no be able to prevent all the heals and it *could* become annoying.

most of the cases you'll only need the paly however.