I purchased a Godaddy SSL cert for $15 today and spent a lot of time tinkering with getting different parent portals to use the same ssl cert and I was able to get it working but I am not sure what I did that made it work but this is what I think did it (domains mentioned here are fictional)
I got a cert for secure.domain1.com
domain1.com existed before as a parent portal
created a parent portal called domain2.com. Created a page in it and copied its full url. In the url I replaced 'domain2.com' with 'secure.domain1.com'. Went to domain2.com site setting and entered the new full url as the SSL url for the site.
Did the same thing for domain3.com.
I tested both pages on a different and the pages opened without any messages and the lock was displayed.
I got a few messages about the cert belongs to another domain. I kept playing with the http aliases and ssl urls and it works now.
I will have to test all this again and test if this solution works only for a single page in each portal. Also test if ssl url can be truncated to secure.domain1.com/xx where xx is page name.
Perseverance pays off.