Login page on one domain to log in to pages on another domain

Don't ask, not my idea.

I'm assuming this is a no no.

Basically I have been working on a big project that uses a common database to serve up data to a few different websites.

For one reason or another various log in and database admin parts have been built on three different sites.

I had thought we had now settled on moving everything over to the site which is kind of the umbrella site for the others. Which I thought made absolutely good sense.

So having done a whole heap of that, the site owner then suggests having a log in page on another site (so as not to confuse the people registered), but then take them to admin pages on a different site.

I'm assuming this is a big deal breaker. Which suits me, as I think it would be less confusing with all this stuff on the same site, with logos and text to explain why it was there.