Two long, straight, parallel wires carrying currents exert magnetic forces on each other because each wire creates a magnetic field that acts on the current in the other wire.
The direction of the force depends on the direction of the currents:
The force per unit length between two long parallel wires increases with both currents and decreases as the distance between them increases. In practical terms, if either current is doubled, the force doubles; if the separation is doubled, the force halves.
Use the simulation below to compare how the direction and size of the currents affect the force between the wires.