Yeh my BIOS also says dual enabled a line like:

DDR Memory Clock: 333MHz (Dual-Channel Enabled)

If yours doesn't then look at the slots that the memory is installed in. The memory chips have to be the same size and clock speed (PC2700-333MHz, PC3200-400MHz, etc). Also, they have to be in certain slots. Consult your motherboard manual to find out which slots you have to put the memory in.