How the hell did I spell rested wrong twice! ugh :(

You receive 10% or 20% (depending on the items you have) of the amount awarded per kill (or per quest though that is irrelevant for rested state)

So for example,

Normal state. Mob gives 500 xp. +10% is 550 xp (one heirloom). +20% (2 heirlooms) is 600 xp total.

Rested state. you get 500 xp +10% is 550 xp. +20% is 600 xp total. Rested Bonus is equal to the XP received per kill.

(If you enable xp gain in chat you'll see a (+500 XP rest bonus) when it is showing you your gains)

Without rest (assuming 500 xp base as an example)
No heirlooms: 500xp per kill
One heirloom 550 xp per kill (10% bonus)
Two Heirlooms 600 XP per kill (20% bonus)

With rest
No heirlooms: 500xp per kill +100% rest bonus = 1000 xp per kill
One heirloom 550 xp per kill +100% rest bonus = 1100 xp per kill
Two Heirlooms 600 XP per kill +100% rest bonus = 1200 xp per kill

Or thats how it seemed to work for me.

Yes, the bonus XP from heirloom items is multiplicative with rested XP. If you wear both items, you would get 240 XP from a 100XP base mob. Curiously, it seems that you do NOT get 242, which is what you'd get if the two heirloom pieces stacked multiplicatively with each other. But it does multiply (and not add) with the rest bonus.