If you want to find anything in Ghana, you can usually find it on Google Maps. There are many hostels near Tamale; just search on Hostel near Tamale, Ghana and you will see. Many hostels will negotiate a monthly rate, and in your case, a two month rate significantly lower than their daily or weekly rate. I contacted The Crystal Hostel in Accra several months ago and they were more than willing to negotiate a longer term rate. I have had trouble making contact with businesses in Ghana, and trying to talk on the phone and understanding the Ghanaian accent is sometimes impossible,so, what you can do is reserve a room in a suitable hostel and negotiate a rate after you arrive, with that hostel or any of the many others nearby.