Domain mapping for urls

Hi,

We have 2 domains.
We want to map each of them as the root for urls.

For example :

With the first domain domain1.com :
/%postname%/ is equivalent to /blog/%postname%/

With the second domain domain2.com :
/blog/%postname%/ is redirected to domain1.com/%postname%/

Is there a way to do this with a plugin or by editing the htaccess file?

Thank you everyone.