the 2nd may not activate at all
the 2nd may initially activate, then days/weeks/months later you will get a message that you have more than the allowed active installs
and will prompt you to enter a new key or will be deactivated in a number of days.
nothing may happen and you continue to use both in "violation"
personally, the second one happened to me a few years ago
after like a month of 2 installs using the same key