Linking multiple blogs together

ALRIGHT SO I HAVE CREATED A WEBSITE WITH ESSENTIALLY 3 BLOGS TOTAL

I WAS WONDERING IF IT WERE POSSIBLE THAT THE REGISTERED USERS THAT SIGN UP THROUGH MY ‘MAIN’ BLOG WILL BE ABLE TO CLICK LINKS LEADING TO THE OTHER TWO AND STILL REMAIN LOGGED IN? RIGHT NOW WHEN YOU CLICK ON A LINK THAT GOES TO ANOTHER BLOG IT SAYS YOU’RE NOT SIGNED IN AND IF YOU TRY TO RELOG IN IT SAYS INVALID BECAUSE YOU’RE NOT RECOGNIZED ON THAT BLOG.

URL:

http://www.partiesatmyschool.com

thanks guys in advance…