Hi fatboy,
I'm not going to try to do a guide in full first off here, but this would all start by re-setting the HDs with new bootable partitions perhaps...?
Or just use the two for different purposes:
A600 for WHDload (ECS games only you know) and other things
A1200 - perform a full re-do on this machine, make a clean AOS3.9 install.
Nice machines...