I'm running two external screens (one thunderbolt, one via the hdmi port) on unpatched 10.3 without any issue on my mbp... I can use the laptop display at the same time also (usually don't -- but I have to close the lid before I start X otherwise it'll stay on) ....?
Is it just your model? It;s behind my desk but it's some cheapo thunderbolt->hdmi dongle made by "neeto" apparently, works well enough..
[ 131.865] (--) NVIDIA(0): BenQ GL2450H (DFP-2): connected
[ 131.865] (--) NVIDIA(0): BenQ GL2450H (DFP-2): Internal TMDS
[ 131.865] (--) NVIDIA(0): BenQ GL2450H (DFP-2): 340.0 MHz maximum pixel clock
[ 131.802] (--) NVIDIA(0): BenQ GL2450H (DFP-0): connected
[ 131.802] (--) NVIDIA(0): BenQ GL2450H (DFP-0): Internal TMDS
[ 131.802] (--) NVIDIA(0): BenQ GL2450H (DFP-0): 165.0 MHz maximum pixel clock
[ 131.865] (--) NVIDIA(0): Apple Color LCD (DFP-3): connected
[ 131.865] (--) NVIDIA(0): Apple Color LCD (DFP-3): Internal DisplayPort
[ 131.865] (--) NVIDIA(0): Apple Color LCD (DFP-3): 480.0 MHz maximum pixel clock