Testing accuracy can be doubled by using a double path method. This as its name implies, passes light off the mirror surface twice thereby magnifying any faults. The point light source is positioned closer to the mirror at the focal point.
A large optical flat with a central hole reflects the light back down to the mirror for a second pass. The optical flat should ideally be as big as the mirror under test.
We generally apply this test in the later stages of figuring for mirrors of 300mm or over. There is some more information on how the Double Pass Null test operates on the page of this website regarding Interferometers.
Double Pass Method
The two mirrors used in Cassegrain systems may be initially tested as separate entities although it is standard practice to carry out a Double Pass test of both components together.
The point light source position on axis is often more critical, due to the longer effective focal lengths involved, and a beam splitter may be used to view the image formed. An optical flat at least as big as the primary mirror is essential to test Cassegrain systems.
A slight disadvantage appears to be that the centre of the primary mirror cannot be tested by double pass because of the hole in the optical flat.
However a Newtonian has an elliptical flat and a Cassegrain has a secondary mirror that obscures the central area of the primary in use. So there is not really any disadvantage.
Testing of a large Newtonian or Cassegrain system by double pass is quite easy for the amateur if he/she has access to an optical flat of the same size as the primary mirror. Perhaps your local Astronomy society has one?
However optical flats are actually a lot harder and more expensive to make than a parabolic mirror of the same diameter. As a result, - there are very few optical flats of over 20" diameter in the UK today.
(Although Oldham Optical have one of course!)
Note: Cassegrains do not usually have parabolic mirrors, but the Double Pass method can be used irrespective of the individual conic sections used on the primary and secondary mirrors.