In this work, we have extended the concept of constrained motion control of robots to surgical tasks that require multiple robots. We present virtual fixtures to guide the motion of multiple robots such that spatial and temporal relationship is maintained between them. At the same time, our algorithm keeps the surgeon in the control loop. Moreover, we show that these virtual fixtures allow bimanual tasks to be completed using input for a single robot. That is, the user requires only one hand to cooperatively control multiple robots. This reduces the cognitive load on the surgeon and makes multiple-robot setup for surgery more relevant. We demonstrate this architecture by using an example of manipulating a surgical knot to position it at a target point. Significant improvement is observed in the accuracy when bimanual virtual fixture assistance is provided. Moreover, the accuracy when using a single input from user is similar to the accuracy obtained from bimanual assistance.