Unfortunately, the vast majority of Hinduism is still out of reach of Western converts - and, for that matter, most Indians. Why? Because only a small % of the scripture has been translated, and the oral transmissions also remain out of earshot.
The guys who really should know a lot are able to dedicate their entire waking & sleeping/dreaming life to learning the intricacies of sadhana (spiritual practice; worship). The picture of the man in your profile - a sadhu, should be one such, although unfortunately most sadhus in India now are little more than beggars or, worse - thieves and miscreants.
There are a lot of sects, a lot of cross-pollenation between them. Within each sect, there are a lot of of different methods to be known and synchronized, if one really wants to study and practice in the way it was intended to be.
To return to the subject of Harihara in light of the above, one can see how deep the undercurrents go beneath the surface. There is tremendous detail - so many mantras, so many vital concepts to charge one's practice with - such as the rishi, the shakti, the meter, and so forth of the mantra, the laws of sound & grammar as they relate to the theory of mantra which the mantrin should be aware of, the dhyana the mantra gives rise to, and its corresponding yantra. It boggles the modern mind the degree to which these yogis were able to virtualize a spiritual cosmos within themselves.
Within this bewildering array of practices, there may well be quite well defined Harihara sadhanas and particular cults dedicated to Harihara.
Here's another useful reference:
Encyclopaedia of the Śaivism - Swami Parmeshwaranand - Google Books
What, may I ask, is your interest in Harihara as ishtha-devata?
