Here are the simple steps you need to follow to enable blog cross-posting:
STEP1: Login to your Multiply.com site. Go to SETTINGS and choose Posting via Email. You will be prompted by a dialog box to create your pin. An email address will be created. The email address would be in this format: pin@yoursite.multiply.com.This is the address that you can use to publish a blog post in your Multiply.com site via email.

STEP2: Copy or remember the newly created email address.
STEP3: Open your Blogger.com account. Go to Settings tab and open the Email menu. Copy-paste or type the email address that you have created in Multiply.com.

STEP4: Save your Blogger.com settings.
Every time you create a blog post in your Blogger.com site, it will automatically be republished in your Multiply.com site.