SCT controller working the North complex into LAX experienced loss of separation when issuing a descent clearance and failing to note conflicting traffic inbound to the South complex during simultaneous procedures.

Narrative

LAX was reporting MVFR for the majority of the day; and SCT (LA Area) had simultaneous ILS in effect. Upon returning from a break; I was assigned to the North LAX final called Stadium Radar (STAR). Traffic for the session was moderate to busy and constant. I was sequencing two for a straight-in ILS; two on the downwind and two props from the north flow over DARTS. Traffic was beginning to wind down when I was sequencing the first prop from the north to follow a B737 on the downwind and was laddering it down over the B737. I was also laddering this prop down over a heavy jet that was going to follow it. I turned the 737 base and turned the prop out 20 left for spacing. I then turned my attention to the heavy and the other prop as I had to get them sequenced in front of the two straight-ins. On my scan; I looked on the South final and did not see anyone straight-in; so I turned the prop on a high base and then went back to sequence the heavy and the second prop. When I looked back at the prop on base it was now descending out of 4700 and when I began the turn to final; I saw a CRJ at 5100 straight-in for the South complex. I asked the prop if he saw the CRJ and he stated he was IMC. Separation was lost. This situation was fully and completely my fault as I know better than to turn an aircraft on a high base (base turn in this situation is supposed to occur at 2500) without being absolutely 100% positive that the south final is clear when I turn base. This is a perfect example of your vigilance and attention to detail must remain at peak; even (or especially) when traffic begins to die down. Recommendation; do not take on more than you can handle. This event should not have occurred as it was my own fault; I simply missed the aircraft on the South final in my original scan. Be 100% vigilant 100% of the time; especially when busy and traffic begins to wind down.