You can use "import" from the backstage menu to import from TW "A"
into your current TW "B". You will see a list of tiddlers to import.
Just select the ones you want.
For more sophisticated, large-scale import/export, get the
ExportTiddlersPlugin from
http://www.tiddlytools.com/ . This plugin
will allow you to select the tiddlers you want to export by tag or
other criteria to a pure store file, which you can then later import
into your target TW.
Mark