I'd bet it's because you can't piggyback the AGA versions like you can the ECS ones.
DIP makes everything so simple.
Correct - the ECS version is a totally different idea. While the AGA version takes it's video data from the Lisa output, the ECS version had to take all data from the "other side" of the chip, as mentioned above. The reason for that was the dirty SHires output of the ECS denise, which is drifting a lot over temperature. There was no way of sampling the Denise output without having to add an adjustment screw.
Such an adjustment screw was not an option - I wanted the product to be compatible without the retro-like pixel flickering that is caused by a bad adjustment. The only way to get around that was to implement a full Denise, as the chip bus runs at a stable 3.5MHz, and it doesn't drift over temperature too much.
That said, it should be clear how we're splitting the screen: Imagine two Denise chips taking the same data, but turning it into different pictures. Although the actual implementation is different, you can imagine one Denise using bitplanes 0 and 1, while the other Denise is taking bitplanes 2 and 3. This can of course only work if the resolution is the same on both screens.
Reality is slightly differnt, as we have the advantage of the full frame buffer and detailed knowledge about the mouse pointer position (chip registers are 100% known to the FPGA), but we do NOT intend to write a WB emulation. Everything must look to the Amiga OS as if it's a single monitor. In this case, it's just twice as wide.
Jens