People will tell you no, and that you should go for the domain alias option and rely on SNI to handle the multiple SSL on the same ip.
Yes, this will work most of the time if your customer visiting the site has a newish windows version with a modern browser, but what about all these people still using windows XP? Can you afford to loose that 20%? (source: http://www.w3schools.com/browsers/browsers_os.asp)
How come out of all these people using opencart for business don't know how to do it...or do they, but they don't bother telling it to the rest of us? No more!!....I will tell everybody!!

For a business website I wouldn't be too comfortable relying on the SNI and I rather do it the proper way! Now, we all know the server can do it but using plesk adds some limitations...which we need to overcome.
Where there is a problem, there is always a solution...some old guy once said.... so here it is:
THE RECIPE (Opencart multi-store alla Plesk)

You will need:
[*]A Plesk powered VPS/Dedicated server with root access.It may work in a shared hosting using htaccess, but i have not tested it and i don't have time to do it
[*]Putty.exe (free)
[*]Filezilla or other FTP program.
[*]A text editor....like notepad++.
Step 1: Create 2 or more domains and assign a dedicated IP to each one of them, but not add them in a service plan (select none).
Step 2: Set up the main domain.
Go to the domain control panel, and then domains & settinds --> Advanced Operations --> website scripting and security.
Set: PHP support (run as fastcgi)
Install your opencart and test it!
If a session error appear when loading the site, Go back to the domain control panel -->domains & settinds --> Advanced Operations --> website scripting and security (php settings tab)
SET: session.save_path = /var/www/vhosts/Your_primary_domain_here.com/private/whatever_you_want/
of course now you need to create that dir using Filezilla and set permissions to 700.
Added benefits:

Fastcgi is more secure and will also solve the permission problems people have.755 will work just fine. (no need to set everything to 777 which I believe is unsafe!!)
Step 3: Set up additional domains:
Go to "domains & settings" --> Advanced Operations --> website scripting & security.
Set: PHP support (run as fast_cgi)
If done for the main domain also set sessions to the same directory:
SET: session.save_path = /var/www/vhosts/Your_primary_domain_here.com/private/whatever_you_want/
ALSO SET: open_basedir = /var/www/vhosts/Your_primary_domain_here.com/httpdocs/
Step 4: Use the text editor to make a file called: vhost.conf and add the following....
<IfModule mod_suexec.c>
SuexecUserGroup "primary_domain_user" "psacln"
</IfModule>
DocumentRoot "/var/www/vhosts/Your_primary_domain_here.com/httpdocs"
<Directory /var/www/vhosts/Your_primary_domain_here.com/httpdocs>
<IfModule mod_fcgid.c>
<Files ~ (\.php)>
SetHandler fcgid-script
FCGIWrapper /var/www/cgi-bin/cgi_wrapper/cgi_wrapper .php
Options +ExecCGI
allow from all
</Files>
</IfModule>
Options -Includes -ExecCGI
</Directory>
Step 5: Use the text editor to make a file called: vhost_ssl.conf and add the following....
<IfModule mod_suexec.c>
SuexecUserGroup "primary_domain_user" "psacln"
</IfModule>
DocumentRoot "/var/www/vhosts/Your_primary_domain_here.com/httpdocs"
<Directory /var/www/vhosts/Your_primary_domain_here.com/httpdocs>
<IfModule mod_fcgid.c>
<Files ~ (\.php)>
SetHandler fcgid-script
FCGIWrapper /var/www/cgi-bin/cgi_wrapper/cgi_wrapper .php
Options +ExecCGI
allow from all
</Files>
</IfModule>
SSLRequireSSL
Options -Includes -ExecCGI
</Directory>
Step 6: Add these files in all of the additional domains directories : var/www/vhosts/your_additional_domains.com/conf/
Step 7:
Run putty.exe and login as root.
If you only have one additional domain type in:
/usr/local/psa/admin/bin/httpdmng --reconfigure-domain your_additional_domain.com
OR
If you have many additional domains type in:
/usr/local/psa/admin/bin/httpdmng --reconfigure-all
Now restart it by typing in:
service httpd restart
Step 8:
Go to your opencart admin and setup the additional domains.
DONE!
Now you have one OC installation but each store has its own IP and can have its own SSL as well!
No more SSL warnings!!
I wish I could get paid for the time it took me to write all this.....

Enjoy!!!