Assigning request to technicians

Dear All

I have 3 Technician(Technician A, Technician B, Technician C) with 1 IT Manager(Technician M) that all of them have been set as a technician and I have 4 Sites( Site A, Site B, Site C, Site D)
I set Site A for Technician A, Site B for Technician B, Site C for Technician C and Site D for Technician M.
If Technician A wants to assign one of his/her request to for example Technician B, Site B should be associated to Site A before if not assigning is not possible.
On the other hand when a requester from Site A wants to write a request he/she can see both Technician A and Technician B which I don't want,
Cause in this case requester can see Technician M in the list,and if i dissociate Site D from Site A or vice versa then Technician M and Technician A cannot assign their requests to each other

Is there any way to help me to solve this problem

Thanks in Advance
Negin