Haste
 

If you have 2 haste items do they stack?

Depends on the type of haste. Read this, it will clear it up for you.
http://wiki.project1999.org/Haste_Guide

Wearing a Cloak of Flames (36%) and FBSS (21%) will mean you have 36% item haste.

You can then get buffed haste, which adds with your highest item haste until the haste cap for your level.

The guide linked above goes into great detail, but short answer is only the highest haste worn will apply.