Solution
The frame grabber transfers each image into an acquisition buffer in system memory. This image buffers are created in the Image Manager for Grab Acquisition.vi . When using ring buffer, the buffers are created in IMAQdx Configure Ring Acqusition.vi. (If you want to know more about buffers, refer to
Choosing an Acquisition Mode )

When sessins of multiple camera images acqusition are in a VI, each session must ensure that the VI creating this buffer is used independently. Otherwise, separate camera session will share a single buffer. This means that while camera 1 is grabbing and saving, camera 2 must wait until it is done. As a result, the FPS(Frame Per Seconds) that was set up in each actually decreases.