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.
Hey there, Data Center community! I'm Martyna Wojtas and I am the Product Manager for Bamboo Data Center. I’m excited to share that Bamboo 9.0 is now available. We purpose-built this platform to help...