They are essentially two different purposes, even if combined.
The Statement of Purpose is about you trying to help the goals make sense, and demonstrate you are genuine. The LoE is more about focusing on the exact issues that the agent has, and addressing them in a concise, defensible manner.
As an example, a LoE might address an issue where they felt your total funds in PoF were insufficient. Your response could be something like "My school considers 12 credits full-time, and taking 12 credits per semester instead of 15 drops the cost significantly. Additionally, I have a roommate lined up which will reduce my rent from $900/mo to $450/mo. Additionally, the tuition and fees include a U-Pass, which means I have unlimited public transit access. This means no parking fees, no gas, no auto insurance. Given that, my PoF exceeds $10,000 plus the first years expenses, as required. My parents will be contributing additional funds for further years".
Such a letter would do a great job of explaining why you know the funds you have are enough for the tuition and expenses, but the officer thinks they don't. You would not put that kind of statement in your SoP.
Your SoP might explain why someone with an engineering degree is making a transition to law, however. Perhaps businesses in your home country are excited to work in Canada, and the TPP and other free trade agreements can bring great opportunities. As a Canadian lawyer, you could assist companies back home in obtaining Canadian patents, and your bilingualism and experience in engineering will be invaluable for patent work. A SoP like that helps contextualize your actions, and let the officer get in your head, and can be done before specific issues are raised by him.
You can merge them together in later applications if you want, but it's often better to separate them and make it very clear that these are the rebuttals to the officer's conclusions. Make it easy for the next officer in line to see exactly why that officer was wrong.