I have a board that does not work if it detects that the processor is core 2 duo how can I make the core 2 duo to be detected as a single processor?
Assuming you are talking about a motherboard not accepting a Core 2 Duo processor...
The most likely scenario is that the motherboard is not compatible with Core 2 Duo processors. This is not due to the number of cores but the type of processor not suiting your motherboard. There is no trick to make a Core 2 Duo look like an older model processor.
Check your motherboard manufacturer's web site for a list of supported processors. Depending on the age of your motherboard, it may be as simple as a firmware upgrade to support Core 2 Duo or it may not be possible at all.
To get a better answer here you'll have to provide the manufacturer/model of your motherboard and the model number of your Core 2 Duo. The current BIOS version on your motherboard would also help.