keep domain alias

Hello,
problem: i have page http://www.something.com, also have the domain alias from the http://www.somethingsomething.com. Both domain is lookin on one WP system.
Everything work great.
But i need: if someone came from http://www.somethingsomething.com and click on item in menu he is redirected to http://www.something.com/nextPage. I need to keep domain from which he came. so the correct will be http://www.somethingsomething.com/nextPage

Can you help me with these problem please?