Share same domain for wp-admin but for different website

I have site like

• http://www.foo.com
• http://www.bar.com

And I’ve a wildcard SSL cert *.example.com, instead of buying two SSL certs for the above two domains, can I re-use my existing wildcard SSL cert for wp-admin?

e.g.

• http://foo.example.com
• http://bar.example.com

Only for wp-admin for better security, is it possible?

Yes, you can do this and it does not matter if plugins are different for the two sites, as you are not not using the same tables or anything.

Still, if you use plugins that are badly written and build resource urls on top the home url, these will have issues (404 errors mostly).