permanent primary domain name and additional domain name

[imperialbrickwo]

Hello. This is probably a very silly question, so forgive my asking... I've just bought the BT professional webhosting and got 2 domain names as part of the package. I'm just about to activate this and set the website up and it's asked me to pick which one I want as the permanent primary domain name. Does this mean I can only use one of these domain names? Can I link the second domain to the primary domain? Any help would be gratefully received!

[mydearcosmo]

I also have the same question. How were you able to choose which site should get a primary domain if you are using both and are also important and have different topics to focus on? It might be possible that you only have to choose a primary domain so it will be the first one registered to your account. Is that it?