The Beam Path
In this video we show the beam path of the laser and why at least two mirrors are mandatory to align a beam.
The video nicely illustrates why the degrees of freedom allowed by one mirror are not enough to orient the beam in the right way. It is essential to hit the cage system (the key tool for adjusting the TIRF angle) perpendicular and that the beam travels along the central position of the cage. To enable a controlled alignment of the beam within the cage more degrees of freedom are mandatory, which is easily made by the utilisation of at least two mirrors.
And even if not shown in the video, but other reflecting devices do the same job, for instance dichroic mirrors (a solution which will be needed for multi color setups).