virus protection on 2 computers?

I own a desktop  and a laptop. My  laptop is networked to my desktop through a router connected to my desktop. The only thing shared is internet, a few files,  ans a printer. I have firewall  and virus protection on my desktop. Is it necessary to also have an active firewall and virus protection installed on my laptop also, or does the protection on my desktop protect my laptop? It gets expensive to have it on both machines is why I am asking. Thanks for your  input.