Unbreakable Vow

[foodiedb]

No: HPFGUIDX 147985

Hi all,
I have a few quesitons about Unbreakable Vows:
1. Can a person make more then one Unbreakable Vow?
2. If the answer to question 1 is yes, then can a person make an 
Unbreakable Vow which conflicts with another?  For example, what if 
Snape had made an Unbreakable Vow with Dumbledore not to harm Harry, 
and then later made an Unbreakable Vow with someone else that he would 
harm Harry.  What would happen?  Would the second vow just 
not "take."  Would it still look like a vow was being made to an 
observer?
Thanks,
David