Get it down in writing with a signature by both parties I guess, waivering ownership of the logo and acknowledging the ownership by the other company. Your best bet would be to get the advice of a solicitor.
Any reason you/they want to assign copyright on a logo?
Are they paranoid you might come back and assert usage rights or something ;-). I've been a practicing designer for 25 years and no client and ever asked about assigning copyright on a logo over to them.