If its accounts, if google knows you have 2 accounts and has linked them then only 1 will show at a time, if they dont know then obviously both ads will show at the same time. Of course if both ads are using the same domain then only 1 will show, regardless who owns the accounts.
Be careful though, its against adwords TOS to have multiple accounts using the same keywords, its Double Serving and could get you in trouble, even though in this case you should be ok as the urls are the same.
One more question:
If you have a key phrase in one campaign as broad match and the same key phrase in another campaign as exact match will only one of these ads show at a time?