If your license is a 3-seat license, installing and activating on two (2) instance of OS - considered as two (2) PCs - will use up two (2) of the seats from your license.
To answer your question-- yes, you are allowed to install Internet Security on both systems. You will not need additional license if you have not used up your 3-license seats. Else, you will be asked to transfer the subscription from one (1) PC to another.
Simply install and use the same Internet Security serial number on both systems.
The 3 licenses are installed in 3 different PCs. Now I converted one of the 3 PCs to dual boot. Obviously I used one of the licenses for one of the systems. My question is: Do I have to get additional license for the other system in the same PC?