I love my boyfriends best friend who is also my best friend, what do I do?

NOTHING!
As long as you are dating one it is wrong to lust over another. Now if you were to end the relationship with guy number one and then after a period start dating guy number 2 that would be OK.

I love my boyfriends best friend who is also my best friend, what do i do?
I have so been in this situation and I acted on that attraction to my boyfriends best friend now they are not best friends and I am still crazy about him even though I am engaged... you can read all about it on my question the other guy if you want hopefully it will encourage you to do nothing about it!

Don't confuse love with lust. So you love your best friend? You should! But don't do anything else if you value your friendship and your relationship with your man.