This is a very good question. Firstly, I think you should go for Kolibri instead. You will have a better UI and more recent Khan Academy contents.
If you refer to this post (about Kolibri), you can see that it is possible, but difficult! Another option would be if you install Kolibri on an external device and have the two chromebooks access through that device, for instance a Raspberry Pi. If you do prefer KA Lite, then the installation instructions below are basically the same… but once you have
apt install working, you have to use the KA Lite install instructions.