Is there a way of merging/combining two different images?
You cannot merge two images so you have only one image with the possibilities of both installations. For this you have to make your own installation and install the programs that you want from the other images. But you can run both images on one Raspberry Pi independent from the other. You shutdown the running image and boot into the other.
You can simply change the SD Card, but that is obviously not what you want.
You can enable booting from an USB stick instead of the SD Card. This makes changing the image (aka USB stick with the image) a little bit more comfortable. You can use the official How to boot from a USB Mass Storage Device on a Raspberry Pi 3. But don't do it with a Raspberry Pi 3B+ (the plus is important). It boots from an USB stick out of the box when no SD Card is present. If you follow the doku for a RPi 3B+ then you will loose functionality for GPIOs 22-26 .
You can use a bootloader like BerryBoot or U-Boot. I haven't used one of them but it seems that BerryBoot is much more easier to install than U-Boot. With a bootloader you can install both images on one SD Card/USB Stick and on reboot you get an Dialog where you can select what image to use.
The most comfortable solution is to use a virtual machine that can run different virtual guest machines at the same time. With this you can simply switch between the running guests. But this needs some experience with Linux and can definitely not used on a Raspberry Pi because lack of performance. I tell it only for completeness.
I think using a bootloader is the best compromise.
 Boot flow
I did read a while back a tutorial of creating your own PiCroft image on the picroft site. so in theory you could add PiCroft to Volumio image on the pi, by SSh into it and running commands. not sure if some of the configurations would interfear with each other though. let me know how you get on as I am looking at this too, but new baby so not much time.