Firstly, don't take my response as the final word. I'm just stating what's happened in the past.
Short answer: Do at your own risk.
It's always been mentioned that boosting minigames to alter the outcome of a minigame is against the rules, and you run the risk of being banned, however there's countless people who consistently break this rule and continue to boost minigames without any consequences.
The only time players have been punished for minigame boosting with alts was during the seasonal highscores, where several players were temp banned and had their titles revoked.
If you don't want to take the risk with your account, then I wouldn't recommend boosting at all, for that small chance that Jagex decides to take action. If you see others doing it, then send a report in-game, and someone at Jagex may take action.
Our community has been asking for this rule clarification for years, and JMods all seem to give different answers...
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Success is not final, failure is not fatal. It is the courage to continue that counts.
Also, I agree that Jagex has a tendency to make rules that are not clear or concise which can lead to confusion on what is allowed and what is not allowed.
The 'minigame multi-logging with alts' rule is one of those rules that exists but is not strictly enforced. I do however know of other instances where accounts have been banned for this rule but I am not at liberty to share them.
It should be at ones own risk to part-take in breaking this rule, as at any time Jagex decides to they can ban you for it without legal repercussions.