How Redirect 301 for Multidomains to ONE domain and best for SEO ?

Hi, I'm using two domain for one website. ( My portal's using Joomla Source and my forum's using Vbulletin source ).
EX
My portal : http://main-domain.com/portal and http://second-domain.com/portal
My forum : http://main-domain.com/forum and http://sencond-domain.com/forum
All're pointed to ONE website.

I'd redirected http://main-domain.com.com & http://sencond-domain.com to portal link with .htaccess :

Code:

RewriteEngine on

RewriteCond %{HTTP_HOST} ^main-domain.com.com$ [OR]
RewriteCond %{HTTP_HOST} ^www.main-domain.com.com$
RewriteRule ^/?$ "http\:\/\/main-domain\.com\/portal" [R=301,L]

RewriteCond %{HTTP_HOST} ^sencond-domain.net$ [OR]
RewriteCond %{HTTP_HOST} ^www.sencond-domain.com$
RewriteRule ^/?$ "http\:\/\/sencond-domain\.com\/portal" [R=301,L]

Is something wrong ??? My Alexa Rank's down when I use 2 domain to 1 website, it's so bad :(. I want to use two domain for 1 website and two domain redirect to one domain which is main domain.

My mean :
-http://main-domain.com & http://sencond-domain.com ---> http://main-domain.com
-http://main-domain.com/portal & http://second-domain.com/poral ---> http://main-domain.com/portal
-http:/main-domain.com/forum & http://second-domain.com/forum --> http://main-domain.com/forum

( all link're redirected to domain1.com which is main domain )

And Please help me to have a Redirecting Method to Optimize SEO

Something like this should help you: Changing Domains: Directing Old Site to the New Site with htaccess

Originally Posted by Michael Biddle

Something like this should help you: Changing Domains: Directing Old Site to the New Site with htaccess

Yes, But this's for root domain. My source web is in folder ?

You can place it anywhere you like and adjust the pointing URL.