Good to hear you found it easy to integrate zymkey into your system.
Regarding your specific application, there are many ways to tackle this and we can only advise in broad strokes what a potential solution might look like:
i) On each of your two RPis create an encrypted volume, using zymkey, following these intructions >.
When you have completed this step, then you can direct your recorded video into this volume, which means your video will be encrypted at rest (on the SD card).
ii) Set up a peer to peer TLS link between you two RPis, with mutual authentication. (check out the RPI community on how to tackle this). The TLS link can be configured to achieve mutual authentication of end devices, and will ensure that all “data in flight” is encrypted.
iii) You can read your video from the encrypted volume on RPi_1, transmit if over TLS (encrypted) connection to RPi_2 and into the encrypted volume on RPi_2.
This method should achieve your required objectives in broad terms. Please note that if your RPI’s are physically exposed, then there is a potential attack vector as your data is read out of encrypted_RPI_1, and transferred to the TLS connection, and similarly at the other end. If you use the perimeter tamper detect feature of Zymkey, then you should be able to make your system resilient to such physical attacks.
Good luck, let us know how you get on with your project.