This is a great suggestion. We're always looking for better ways to handle combined/split orders but it does get a little tricky given the amount of scenarios that are out there. For example, if there are discounts included, we're going to need to handle updating/combining a bit differently than your scenario or if something was split. It gets even more tricky when there are multiple splits or combinations in a single order, which can happen frequently.
As there are lots of options in splitting and combining, before even considering providing a more robust updating feature, we want to make sure we understand each scenario so we know how to handle them, so thanks for sharing your need for this! I've gone ahead and forwarded your request to our Product Team so that they can look into this for you. Please keep an eye on this thread for any future updates. Thanks again for the feedback!