I would say if you are working on a project together, something you are going to both take credit for, it may by-pass the copywrite. But sharing, lending or copying would be an infringement. In copywrite world "lending, "sharing" and "copying" are all interchangeable words.
If push came to shove, how will you prove that you bought it "together"? One person bought it (meaning there is one sales receipt) and the other pays her for her half. Technically, one person bought it. The buyer is "sharing", and the other is "giving her half the cost" for the ability to use the pattern, wheather working together or alone. I would think that that may be a problem.
If you are in doubt, don't share, lend, or copy. Copywrite law is such a pot of mud. I would check with the artist or the copywrite holder before even buying the pattern.