Neither the BVision nor a PCI gfx card plugged into a Mediator support this (only some older, Amiga specific gfx card do). You will always have to use two monitors, unless you are willing to buy a tv card and connect the Amiga's native display to the tv card.
That`s not entirely right. though the BVision has no port for looping back the 15KHz RGB out is can make use of the "softwarescandoubler" that is build in the later version of the BlizzardPPC flashroms. This optin is only meant as an emergency measure but it works fine with the bootmenu and the Workbench, as long as you don`t open a graphicscardscreen. I`m not sure about gamecomaptibility.
The other way would be an external Videoswitcher, preferably an electronic one for quality reasons, but that would cost about 50 EUR.
