A/UX 1.1 and 2.0 on the same drive: How?

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I have A/UX 2.0 on partition s0, swap on s1, a few user systems on s2, s3, 
and s4, and A/UX 1.1 on s5.

How do I boot A/UX 1.1 on s5? This is what I do: I run A/UX startup, click
cancel to get me a # prompt, type in pname -a to recognize partition s5 (cos
it's in /etc/ptab), change (default)/ in the preferences to (0,0,5)/. Then
boot. The right kernel gets booted but then I get a panic "can't mount root".

I can't think of what to try next. Any ideas? The panic comes just after
the message about I/O acceleration so the device drivers have been initted
OK.

Thanks,
Tony Cooper
tony at popserver.stanford.edu