I've had a quick look at the picture..
It looks like you could add either a tape or cd head unit in addition to the empeg... =)
However if the Mach units are not linked in some way that means they work as a pair, and are just linked one into the other, you might only have to replace one of them with the empeg, and not get a second head unit... =)
(Anyone familier with the way they are linked?)

My Fiesta also has 2 din sized slots (But only one had a head unit), so I run a JVC CD / MD in dash player (mainly as the volume control) and the empeg into the line in...

Jazz
(List 112, S/N 00030, 4 gig blue)