This review may contain spoilers. I can handle the truth.
The film explores identity, the merging of consciousness, and shared experience. If our sense self is tied to someone else, who are we without them?.
Also an interesting aspect I noticed, is that technology is quite a prominent theme in David Cronenberg films. Although this film, is focused on identity it still appears. The brothers made their name through inventing a new instrument, that alters the profession (new technology), at the start of the film. Then at the end, another new invention brings about their very demise, and the end of their lives (the mutant instruments).