1. Link it to your steam account.
2. Install it on one PC with steam online.
3. Once installed, set steam to 'offline mode' which you can do at the 'file' option top right
4. Now log onto steam online on the other PC
5. Install FM13 on that PC
6. You can now play on both, you will need to keep one in 'offline mode' so when you update, do one first, set it to offline mode, then set the other one to online and update.