What version and patch level of QC are you using? When you say a different domain, do you mean a QC domain or a network domain?
If the two QC servers are at the same application and patch level, and you can log in to both from one workstation, you may be able to open two browser windows, connect to each QC system, and simply execute a copy and paste of the Requirements folder from one IE window into the other. I haven't tried it, so i don't know for sure.
Alternately you might be able to export the Requirements to an Excel file, and then you the Excel Add-in to import those requirements to the other project on the other QC server.
(Opinions and information contained in this post are wholly my own and do not reflect the opinions of my employer.)
you can use the copy paste function, from Source QC to Target QC, as long as system fields & user defined fields match, otherwise you can get a mash of information.
Excel is also a good idea.
However, this will only be valid for exporting/importing requirements. Remember that with either one of the methods above, you will lose all the attachments, as well as any link you might have to test cases.
There are alternative solutions to the above, but what exactly is it that you're looking to do?