Multiple "Security Testing" within a single server?
You can choose same server for multiple Security Testings, but you still would get credits from only 1 of them. Security Testing effect is a replacement effect which replaces access step. You have one access step per run and it can be replaced with any one applicable effect of your choice. So they all would trigger same time, but only 1 will get to resolve.

Moreover, other Security Testings wouldn't work on subsequent runs on the same server this turn, because it woudn't be a first successful run anymore and they can't meet their trigger.