You need to buy 2 PRO licenses or one ENT license to install twice. If you have a PRO license already you can upgrade it to ENT. If you are not working with a partner already we can help with that.
sorry, I try again to formulate my question (I am a 3cx partner):
I bought an enterprise license.
I got only one license key.
Can I use this license key I got to install the second instance or did something go wrong and 3CX missed it to give me the second key?
Gotcha. If you have an enterprise license already then you can install it twice. It's flagged in the backend to allow two active instances with the same key.
To my knowledge, you are not allowed to install 2 different 3CX Installations with one ENT key (eg. 2 different FQDN´s). The ENT key is for redundance of your primary installation or to get the extra functionality given by a ENT license. - however I might be wrong