"Thank you, Andrew and Iain."
Translation:Tapadh leibh, Anndra agus Iain.
Tapadh leat is singular, whereas tapadh leibh is plural. Since you are speaking to two people in this sentence, you have to use tapadh leibh.
On the other hand, tapadh leibh is also used as a formal way of saying thank you. You would use it when speaking to your elders or perhaps someone you don't know very well i.e. when you want to be respectful to the person you are talking to.
Whether you use math or mhath depends on the gender of the noun it is describing. Broadly speaking, masculine nouns take math and feminine nouns take mhath.