The code will currently only let you create valid orders, which includes checking previous orders, cargo, etc. so you can't unload something you don't have for example. That means any single template has valid orders.
However, it is isn't as simple as checking the first order of template B vs last order of template A because there are a lot of checks before the potential order list is created. There are 110 potential orders and you normally see a small subset. The existing order check code wouldn't be able to handle an order that would not normally have been an option in the first place.
Ultimately, I don't want to spend time chasing bugs caused by incompatible orders that I will never be able to find, so it is much simpler to avoid concatenated templates.