MWH Controller described unsafe event when during operations from the Tower Cab; the radar range was changed by the Tower Controller as a TRACON conflict was developing; reporter alleging combined operations unsafe.

Narrative

I was training a fairly new Developmental with Aircraft #1 on a 180 degree vector climbing to 4;000 FT; and Aircraft #2 on about a 100 degree vector descending from 5;300 FT to 3;000 FT. The descent clearance to 3;000 FT was given with about 9-10 miles lateral separation; however; the courses were converging. I asked the Developmental what kind of separation he had to bring to his attention that he needed to do something. I waited about 45 seconds for him to formulate his plan. At this time; the conflict alert sounded for another situation to the east-northeast between 2 aircraft in the Tower's airspace. The Local Controller started to change the display from a 60 mile range to a 30 mile range to a 10 mile range to observe the conflict alert situation. He then returned it to 60 miles; and I was about to issue control instructions to Aircraft #1 and Aircraft #2 when the Local Controller began adjusting the range again. I stated we need control of the radar again; to which the Local Controller laughed. I stated we were about to have a deal and needed control of the radar. I then instructed Aircraft #1 to turn right heading 270 degrees; and Aircraft #2 to turn right heading 180 degrees. Aircraft #1 and Aircraft #2 did not get any closer than 4 1/2 - 5 miles laterally; however; Aircraft #2 was still descending and I did not yet have vertical separation. This is a safety issue; in that our local management has directed us to have 2 controllers in the operational areas; which precludes having our TRACON open. This is forcing 2 Local Controllers and 1 Approach Controller to share 2 D-BRITE scopes in the Tower Cab. There are no procedures for who controls which D-BRITE scope; and in this situation it just about caused an operational error.