You can't just parallel all the connections from the existing 6264, you also need to take the relevant chip select lines from the edge connector. There's also a write protect switch that ties /WE high if the switch is set to protect mode (and switches /WE through from the edge connector when not in protect mode)
I think I scribbled down a schematic at some point, send me a message with your email address and I'll try to find it and send over.