I do a rather muddy race each spring on front and rear 1.5 slicks in an effort to improve my speed on the paved sections (less than a third of the total distance). I slide around a lot but it (more or less) works.
I have also done the same race on semi-slicks (old Ritchey speedmax, 1.9, I think) and had a slightly slower time, with a lot less near-crashes.
A semi-slick on the front and a slick on the back is a decent compromise. I tried this mixed set-up one year and had about the same time as with both slicks. Looks odd, but it's not a bad idea.