The condition needed for sparks is difference in potential. So it is not mandatory that when two current carrying materials brought together to create a spark.
If the two wires are at different voltage levels, a short circuit is created when they touch each other. That creates the spark. To explain in detail, you should be familiar with the concept of dielectric strength. Air has a dielectric strength of 30 kV/cm. That is, if two conducting surfaces are placed 1 cm apart from each other, and subjected to a potential difference of 30 kV, the air between the surfaces breaks down and allows current to pass through in the form of a spark. If you reduce the gap between the surfaces, a lower potential difference is sufficient to break the air down.
When you bring the two wires closer to each other, at one point, the gap is so low that the air between the wires is ionised and it breaks down, allowing a spark between the two surfaces. So, the spark actually originates before the wires even touch each other.
When two wires of equal potential touch, there is no spark between them.