Hey @Allison1,
Welcome to Camunda!
There is no need , but possible…, to have another engine to run Task A independent of Task B. In your case, it is perfectly fine to use Springboot Camunda and one engine.
So by default when you mark a Service Task as Async it is marked as Exclusive. This means that the Job Executor/Thread will try to avoid executing such Service Task in parallel as another job in the same process instance. To change this behavior you could uncheck the Exclusive check box so that the Job Executor picks up this job. This will make the Job Executor execute the Service Task even if another job of the same process instance is being executed by another thread. This would allow you to run Task A independent of Task B.
As a heads up make sure you are aware that if you uncheck the Exclusive check box you might run into Optimistic Locking Exceptions. See this forum post that references such exception. Optimistic Locking Exception will cause the Service Task to retry itself so take this into consideration when implementing your business logic.
Test this option out and please get back to me with your result.
Regards,