In both instances, both alleles are expressed. How they're expressed is different.
In incomplete dominance, the phenotype resembles a sort of blending of the two alleles. That is, if you have an allele for red petals and and allele for white petals, you end up getting pink petals with incomplete dominance.
Co-dominance is when both alleles are expressed separately. Human blood type is a good example of this. The A and B alleles are both expressed, so you get the AB blood type. In the petal color example above, you'd get a flower with white and red splotches.