I don't think it is feasible to have multiple kernels installed on an ISO. Not only the space but the tools used to generate a Manjaro ISO does not support multiple kernels.
With 18.1.11 - the naming of ISO images has changed to include the kernel used for a given ISO so you - just by looking at the filename - know which kernel is used. Then intention of this naming scheme has been made for this .
I am am maintaining the Openbox Edition - if you need a newer kernel - check the Openbox download section at OSDN.
Manjaro Architect is available on all ISOs, making it possible to install whichever edition you like using any bootable Manjaro ISO.
Openbox will often be available with the latest stable kernel.