This is a very good question. The absolutely proper way to write this would be "Please let me know to whom I can speak for assistance." If you're writing something which requires you to be formal, then this is the correct way. However, most native speakers (and I'm one) tend to let the prepositions hang - as in, "Who I can speak to for help?" So, if you're writing more casually, it's perfectly okay to use this form. You're right, though - try not to have two of the same prepositions sitting side-by-side, as in your "who I can speak to to get any help." Better to change one as I did in the last, informal, "Who can I speak to (or with works as well) for help?