I never said whether or not there is separation. I am asking why members of Congress can pray at work on Federal government property.
Because not all laws apply to Congress, ex: Healthcare. The big ones you will usually hear about are Freedom of Information Act, EEO, and OSHA - none of which apply to Congress (though they will often respond to FOIA requests).
I'm not a lawyer so I can't tell you all which laws apply and which don't, but if you think that just because it applies to the American people, it applies to Congress - well, it just doesn't work that way.