Double nunchaku (or simply "doubles" refers to the use of two nunchaku - one in each hand.
The techniques performed with the nunchaku can be either the same technique synchronized, the same technique not synchronized, or different techniques. Timing is very important to avoid two sticks striking each other.
Many poi techniques can be performed with double nunchaku, including the weave, butterfly, windmill, and many techniques derived from these. Isolations while still possible to a certain extent, are much harder, and poi isolation offer a much better visual effect.