Please help utility to enable HFS to recognize a APFS startup disk!

I have 2 ssd drives, one with El Capitan and one with High Sierra. When I select startup disk from High Sierra to El Capitan, I can boot into El Capitan, but in El Capitan I can not boot or select High Sierra as startup disk, it doesnt see the new apfs. I was hoping this utility APFS Retrofit Kit for macOS would do the trick but its been discontinued...... Where is a solution? Now to boot back into High Sierra, I have to physically remove the El Capitan SSD.