Assign 2 resources to task without changing duration

I have two resources. One is a person and the other is an instrument.

The person has manual tasks that he does by himself.
The instrument runs by itself to complete an automated task.

And: The instrument has to be cleaned by the person.

When it is being cleaned, both the instrument and the person are 100% busy. It takes 15 minutes to clean the instrument. How can I set this up so that if an instrument and a person are both assigned, they are both 100% busy and the duration doesn’t change?

@ljw It sounds like you may need to set your Resource Allocation option to “Adjust task effort” for this task - setting up something like this:

I hope this helps!