A PCT Controller described a potential conflict involving less that precise facility operating procedures during parallel runway operations at BWI.

Narrative

I was instructed to relieve the BWIFS position after my scheduled break. During the relief briefing I was told by the out-going controller; that we were issuing instrument approaches to Runway 33L. The controller also told me that the only aircraft on the frequency was Aircraft X (approximately 10 southeast BAL at 30). Before releasing the position to me; the controller issued Aircraft X a right turn heading 280 (from 260). I assumed the position on BWIFS; and proceeded to set up my RADAR scope configurations. Within ten seconds of assuming control of the position; I read Aircraft X the following clearance: 'Aircraft X; you're six miles from GRAFE; turn right heading 300; maintain 20 until established on the localizer; cleared I-L-S Runway 33L approach.'. I observed the aircraft turn right heading 300 and descend out of 30 for 20. Within 15 seconds after issuing the approach I noticed Aircraft Y on a right base heading 250 for 33R at 20. I immediately canceled Aircraft X's approach clearance; called the traffic; then instructed the pilot to turn left heading 290 and intercept the localizer on that heading. I yelled down to the BWIFN positions (three scopes down on my right) and asked 'What is Aircraft X doing!?'; I erroneously remember the controller say 'He's got visual'; although now; I believe he said something along the lines of 'He's trying to get the visual (approach)'. I observed Aircraft Y turn right heading north northeast; and then I reissued the clearance to Aircraft X. BWIFS owns 30-40 in that area. BWIFN owns AOB 20. Who owns 20-30? I recommend that airspace be designated to a sector. BWIFS was running ILS approaches to 33L; and BWIFN was running visual approaches to 33R. I recommend that if the sectors are split; that they both run the same operation. Finals are rarely split when in VFR conditions. I recommend this position is only split when really needed (IFR) AND when they do; both RADAR positions should be side by side (not separated by two other scopes). The previous controller turned Aircraft X on a 280 before I assumed; and therefore put the aircraft in a compromising position. There is no SOP procedure that outlines the sequencing and/or separation responsibilities while running a split final in a simultaneous runway operation. I RECOMMEND THIS BE ADDRESSED IMMEDIATELY! Aircraft X did not see the field while still being vectored on a base leg at 20; instead of being vectored towards the airfield so that I could turn my aircraft inbound.