You can create a Trial Account for World of Warcraft by downloading the trial. If you later wish to upgrade to a full account you will have to buy the game and completely download or install it but you will be able to retain any account information or characters you made during the Trial Account.
It is advisable to find a friend who plays World of Warcraft and creating an account (trial or not) through the Recruit-a-Friend option. If you should ever decide to make a full account, you'll get all of the rewards offered from Recruit-a-Friend and make playing the game more fun for you since you'll be playing with someone you know.
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No you cannot, Wow will only allow you to log onto 1 character per account , at a time.
Yes, you can use any addons that a full account can use. There is no limit.
Yes, making an account is free. You have to pay to play the game, though.
If you mean merging 2 wow accounts so all the characters are available from 1 account, that is not possible. It is, however, possible to merge up to 8 WoW accounts under a single Battle.net account. You can do this by going into your Battle.net account and adding your second WoW account to it. This can only be done if both accounts are registered under the same name. You will still have to log into an account separately to access different characters, but you can start up WoW twice, and log in to a different account with each session. Addendum:The above answer isn't correct. You can certainly merge 2 accounts as long as your name is registered to both. The only exception to this I believe is if one of the accounts belongs to a minor over whom you have guardianship.
A trial account itself is not the issue, it's the installed program (client). Once you have that installed, and you have made an account with a private server, it will work regardless of you also having a trial account at Battle.net.