Simultaneously log in to an external client admin and concrete5?

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Hi all,

We currently have a client admin area where clients manage other things as well as their website. The CMS we're using is custom though and not so great so we're looking at getting rid of that and "upgrading" to C5. I'm not a developer, I'm a designer so I don't know much about this sort of thing.

Anyway my question is: Can we make it so when clients log into our existing admin panel would it be possible to simultaneously log in to Concrete5 as well? So when they hit a link in our admin panel to "manage site" the pages would be editable and not require a second login.

I read a bit about OpenID but I don't think we can use that.

Thanks in advance for any advice.