You're really between a rock and a hard case. I don't envy you there.
However, I suspect that you are not teaching how to use them together, nor in separate times in the same session, so if it came to the crunch, you can use them on different devices.
For those whose workflows tend to relay upon Visio for diagrams to be inserted into documents, as a lot of enterprise tech writing involves, it is a pain to have to use separate systems when trying to make the text and diagrams tie up. But in those environments,
having the latest version is typically not required.