I have a small site that I want to merge with a bigger one. Both on Wordpress.  How can I merge the second one with the first?I know that one solution would be to make the smaller one a subdomain of the bigger one, but I would like the following thing to happen: when I click on a category or a tag, posts from both sites/databases would appear. Something like Smashing Magazine did when it assimilated designinformer.com. The other solution and the one that I would prefer would be to merge the two databases, but I don't know if this is possible. 
I want to combine the databases from two different sites under one URL. How is this possible?
database