How to use iTunes Match with two user accounts on the same computer?
There's a kind of easy way to solve this as I found out recently. All it involves is using the same credit/debit card details on both iTunes accounts.
Yes, a little bit of a nuisance and there's no logic at all as why it would be this way but it solved it for me with my Macbook that I share with my daughter. 2 separate Apple ID's and 2 separate iTunes match accounts on the same Mac but she just pays me in cash if she buys something...