Yes you can. See this man page: http://b2evolution.net/man/collection-cookie-settings

Note: what you can do with cookies is restricted by the browser. b2evo cannot make your browser do things that it doesn't want to do. You need to work within the limits of the cookie specifications.

I also have subdomains and collections, but I have a redirect for each subdomain.so that the subdomains all go to the root domain and then a sub directory, which I think is like a stub, each of which then calls up the specific blog. So I only have to sign in on one collection yet can access them all.

blog1.domain.com redirect to domain.com/blog1/
etc.
in root I have a folder called blog1 with an index.php directing with $blog=number of blog

so just one cookie to the root domain.

Amoun, you could perfectly run your blog1.domain.com on blog1.domain.com without redirection and still have to sign in only once for all of domain.com. As I understand @csaba succeeded.

Merci, Monsieur Planque :)