Now I've DO installed it on 1 single desktop and 1 single laptop.
I want to add one more desktop, what options do I've?
From Common purchasing, upgrading and licensing questions, you can add more licences via here: https://www.gpsoft.com.au/DScripts/licencemanager.asp
Which option more cheap? add a new license to the current one? or buy a new one?
I was not able to find an answer for that question from the links you thankfully mentioned above.
Adding a second licence should cost less than buying another separate licence. It will tell you the price before you pay.