Aha - Biz, you gave me the idea to go look at the Customer Service forums - found this response by a blue:

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Question: So im wondering if im linked to an account where lv70 runs around helping my level1, will my level 1 still get triple experience from questing?

Answer: Should the level 70 and the level 1 character be on linked accounts, neither character will receive an experience bonus, as the mobs being killed and quests being completed may not be trivial (gray) to either character.

Should you be partied with a level 1 (on a veteran account), a level 5 (on a trial) account AND with a level 70 (linked to neither), the level 1 will receive triple experience; however, that experience will be first mitigated by the presence of a higher level character. The level 5 will not gain an experience boost.
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So I guess the key would be to link accounts 2 (pre-existing) and 3 (new trial account), and have account 1 (level 70 toon) boost them both - making sure they stay the same level.