You can use unofficial/unsupported uae_ options to archive this. For example
hard_drive_0=foo.hdf
translates to
Code:
uae_hardfile2=rw,DH0:./foo.hdf,32,1,2,512,0,,uae
uae_uaehf0=hdf,rw,DH0:./foo.hdf,32,1,2,512,0,,uae
Change the 1 to 2 and you'll have your 2 surfaces.