can I merge two iTunes accounts?

You cannot merge two accounts together. You can, however, have songs from both accounts in the same library authorized to play on the same computer.

How would you go about having both accounts logged into simultaneously. Thanks.

You cannot have two accounts logged in to the iTunes Store simultaneously, but you don't have to. Just authorize the first account, then authorize the second. Usually all you need to do is play a track purchased from an unauthorized account and it will offer to authorize; just enter in the appropriate user name and password.

but apparently you have to do the second authorisation over and over again...
Meaning, every time you close down iTunes, you have to do it again...

any chance of avoiding this?

Roger

You don't have to authorise every time at all.
My iTunes uses two iTunes accounts and happily plays music from both and I never have to log in to either to do it.

The only time you need to login to change the account is if you want to access the specific account or purchase music from that account.