Link Displays Between Two iMacs (Late-2015 Retina running Sierra and 2017 running Mojave) Using Thunderbolt-to-Thunderbolt

I'm hoping someone can help me (and that this is possible, considering I just spent an arm and a leg to facilitate this setup).

I have a late-2015 iMac (27" Retina 5K display, running Sierra) and a 2017 iMac (21.5", running Mojave) and am hoping that I can connect them for dual display. I have them connected via Thunderbolt-to-Thunderbolt with the TB2 to TB3 adapter. For whatever reason, I'm not able to use Target Display mode on my 2015 iMac, and I'm suspecting it's because the model doesn't support it. ETA: The other mac doesn't show up in either System Preferences > Displays menu

I'm hoping someone can offer advice as to what I'm doing wrong or a possible workaround for this issue, as I have no use for both iMacs separately but would greatly benefit from using one as an additional display - even if the answer is simply that I can't do it, it will at least allow me to stop spinning my wheels and give up on this pipe dream.

Any suggestions are welcome!

I'm afraid neither of those iMacs is capable of Target Display Mode
Generally it was iMacs between 2009 & 2014 that supported it, no retina iMacs were included.

See Lifewire - How to Use Your Old iMac as a Monitor

Unfortunately, Apple's own KB article on this seems to have recently vanished.