thats actually so simple I can answer that. First thing is first. You need to specify a parent domain that is the main one that all the other blogs will need to point to. (just like in apache)

part 2 (apache or lighttpd) make the two other domains aliases of the first domain. so that the main domain is domain1.com and the other domains are listed under the ServerAlias directive.

part 3 in b2evo now you can go in create the blogs and make them only respond to the url you like. You can do this under the blog settings > url tab. Make sure you use absolute url, I think it's the third option.

Hope this helps.