Can a 21.5 inch iMac (non-Retina late 2015 - 2.8 Ghz, Intel Iris Pro graphics, model 16,2) drive two external displays?

Not according to EveryMac who are usually spot on with their info, including unsupported 'hacks', none of which are mentioned for this model.

This model can simultaneously support the internal display at full native resolution and up to 4096x2304 on an external display in both extended desktop (second workspace) and video mirroring (internal display duplicated on the external display) modes.

I bit the bullet and bought another DisplayPort monitor (Dell U2414H), and sure enough, it works out of the box! I'm currently running a total of three displays - the iMac built in at 1920x1200, the 24" Dell at 1920x1200, and the 27" Dell at 2560x1440.