Kafka camunda multiple instance

i have modified the above process to reflect

my issue here is when i am running a single instance of worker its working as expected but when i run multiple instance of worker then instance1 is picking one worker and instance 2 is completing the task. and instance relies on the variables in the external task of instance 1 . how do i ensure that irresptive of any workers getting picked from any instance , externak task of a specific process instance id is picked ?