It's not currently possible to combine iTunes accounts or to transfer content from one account to another account. You should be able to have both accounts authorised on the computer's iTunes that you sync to (via the Store > Authorise This Computer menu option) - but the apps will remain tied to the account that originally bought/downloaded it, so only that account will be able to download updates to those apps.
Also note that using iTunes match, using automatic downloads or re-downloading past purchases will risk tying that computer (or device) to that account for 90 days : http://support.apple.com/kb/HT4627