How to schedule a resource to switch from a job to another

Hello,

I'm schedulling an engineering job, MEP.

the plan for mechanical as follows:

there are 3 desginers, each will take a division - pluming 1 engineer, HVAC 1 engineer, and FF 1 engineer.

each have a drawing list with assigned hours for each drawing, and WBS is agreed.

FF desginer will be the first to finish, so, I want him to support pluming, but only after he finishes his given task.

then when both FF and pluming finish, I want them to support HVAC.

How can I do that on P6.

Thanks in advance.