Multiple Site running on Single IP and Single SSL

Hi,

Here is our current setup:

Domain name   SSL Certificate   IP           http port    https port

www.a.com       www.a.com          1.2.3.4      80              443  

www.b.com              --                1.2.3.4       80                -- 

We have SSL Certificate for www.a.com only. But while connecting the www.b.com through https like https://www.b.com, its redirecting to https://www.a.com. How to stop this?

Thanks in advance