2 sites on 1 webspace 250/250

[#1]

If anyone could help I would be grateful. 

Ok, so i have 1 .co.uk domain that is registered and hosted with plusnet. 
1 domain that is a .com.ua type domain registered elsewhere (don't ask  ;))
My webspace is the 250mb space x 250mb bandwidth
I have updated the nameservers on the .com.ua domain to ns1.force9.net  /  ns2.force9.net

Now what i want to do, is host this .com.ua domain on my plus.net webspace.  So that ideally both sites can be seen when typing in the relevant url,  but if this is not possible just to have the .com.ua as the only site on my webspace with plusnet.    as the .co.uk is not really used at all but it would be nice to have it as a test site if possible.
Anyone have any ideas?

[#2]

You should be able to achieve what you described, either giving each domain a separate subfolder in your webspace, or perhaps your primary domain in the /htdocs root and the test domain in a /htdocs subfolder. Hosting of your .ua domain will have to be set up with Plusnet if you haven't done that already.
Detailed guidance in what is needed is given in Help & Support, I suggest starting from the hosting top level here, and following the domains link.