Playing Red Alert 3 on Network (CD Key Error)

Hi

My sons are very fond of playing Red Alert 3 and I have been trying to let them play over our local network. However, every time I try to play it throws up a CD-Key error, even though they have different accounts and I’ve bought two copies of Red Alert 3 using different email accounts (and signed in using the different accounts). Is there any way to fix this?

Thanks

Chris