If you really wanted them both in the single foobar2000 profile you could divide the two sources using title formatting via a custom grouping column. I'm only familar using Facets and EsPlaylist for this purpose but it's probably possible with other library display panels as well.
See this semi-related post for an idea of how tabs can separate different parts of a library using either the $strstr() function (with Facets) or query syntax (with EsPlaylist). The basic premise is:
$if($strstr(%path%,MyPathHere),True,False)
Which translates to: 'if the file path contains MyPathHere do True, otherwise do False'. So to include only tracks from one drive you could make a custom column for each tab that displays only that drive's music:
$if($strstr(%path%,C:\),[%album artist%],)
$if($strstr(%path%,K:\),[%album artist%],)
The %album artist% tag could be substituted for whichever tag(s) or formatting you normally group your music by.
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I'll add that for my portable music I simply replicate the same path structure (apart from the root directory) which makes it easier to create playlists since they share virtually the same paths (and there's no need to display the portable copies of the files in fb2k). So with just one find and replace tweak in the m3u I have a ready-to-use playlist that works on the portable device.