I triple boot both my A4000 and A1200 (OS3.9, OS4.0 and MOS PUP) without any problems.
Both miggies have 3 partitions under the first 4GB of the HD.
DH0 = 3.9
DH1 = 4.0
DH2 = MOS
I have Bootpriority = 1 in DH0 and 0 in the other 2 partitions. MapRom is definitely Off!
So the system boots always in 3.9 unless I choose from the Early Startup Menu to Boot in DH1 or DH2. That way I can boot whatever system I like per case.
Only drawback is in 4.0 where you need to go to the Early Startup Menu once more after the new Kickstart loads in order to choose DH1 again, or else it tries to boot 3.9 with 4.0's kickstart = blank screen.
There are utilities like Thomas changebootpri that can change the bootpriority from script but I haven't even bothered yet.
You can check a demo video I took from my A4000 some months ago to check it out
here... (don't mind the Greek though lol)
