If I have two Humminbird units on my boat one up front one on the bow and I want to update the maps can I use the same chip and download/update the new maps to each unit or do I have to buy two chips one for each? Never updated my maps and they are two years old and I was told that you should update them every year? The chips are like 200$ a piece I don't wan to have to buy two or should I just stick with the maps loaded on my units?