Clients usually think that joint ownership of copyright is a good thing; lawyers (in the UK at least) usually think that it is a bad thing. Why?
A number of old English cases (e.g. Lauri v Renad in 1892, Powell v Head in 1879) established that a joint owner will not be allowed to exploit a copyright work without the agreement of the other joint owner or owners.
These old cases were expressly incorporated into the “new” law (i.e. the Copyright Designs and Patents Act 1988).
The principle leads to a problem: where the joint owners get into a dispute, it may become impossible to exploit the copyright work at all, as each joint owner thwarts the other's exploitation plans.
From the commercial perspective, this is less than ideal, as it means that a potentially valuable asset is locked-up.
The effects of this law can be lessened by an initial contract specifying the forms of exploitation permitted by each joint owner, but contractual restrictions are often less certain than statutory rights - and gaps can sometimes be found in contractual provisions, not matter how careful the drafting.
So, before you agree to a joint ownership arrangement, you need to think very carefully about how the relevant copyright work may be exploited in future. You should also consider other possibilities (e.g. one party owning all the rights, and licensing rights to the other party). Whatever you decide, you should ensure that the agreement is properly documented.