> I currently run SunOS and NetBSD on different volumes, and all is well.
> boot disk0 and boot disk3 start the desired system.
> 
> I've been thinking that I might get the performance improvements associated
> with having swap on a different physical device if I exchanged swap
> devices, that is use the netbsd swap partition for the SunOS volume, and
> vice versa.
> 
> Am I overlooking anything that will cause problems?

In general, NetBSD uses the native disk partition format on all platforms.

When I run DUh and NetBSD/Alpha on the same box, I never create a separate
swap partition...I just use the same one for both kernels.

And in your case, you should really use them both on both systems. The
raw partition pager is sort of permenently in ccd(4) mode; it has
always supported interleaved drives on both NetBSD and SunOS.  Or,
change the partition type on the NetBSD swap and put a file system on
it, and then use the SunOS swap for both.