a Bamboo trigger is not what you can use in this context as triggers in Bamboo determine when a build is fired.
Not exactly knowing what the tasks in your plan X are supposed to do, if you rearrange your tasks in a way that b was your last task running in this plan, then by Bamboo's dependency feature, it could kick off plan Y.
Please find details here: https://confluence.atlassian.com/bamboo/setting-up-plan-build-dependencies-289276887.html
Hope this helps.
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