You wouldn't need to give a level to the alt on acct2, as you only get 3x exp if you're equal or lower level than the linked account.

Source: http://forums.worldofwarcraft.com/th...1&pageNo=6#107
Experience would be mitigated by the presence of a higher level character; however, the mitigated experience would be tripled due to the link. Remember, if one of the linked characters is lower than the other, only the lower level one will receive the XP boost.