Something worries me about this idea of EQing two IEMs in order to make them sound identical. I think that task would be virtually impossible without some kind of superhuman memory. Think about what it entails... Constantly switching IEMs (fast enough that short-term memory doesn't clear), somehow discounting any placebo effects you might have from knowing which IEM you're currently listening to, inserting them both in a completely identical manner each time, and recalling the previous response at every frequency within a fraction of a dB. Even with the world's best parametric EQ, this sounds impossible to me. I know I couldn't do it. Even for those that will inevitably claim they can, it's something they'd never be able to prove, because if you took the resulting EQ'd headphones and put them on a coupler, no coupler would match their ears with that level of tolerance. Tiny differences in eartip shape/size, insertion depth, ear canal geometry, etc., can have very sizable effects, e.g., turning peaks into troughs and vice versa.
I agree with
@JohnYang1997 that FR (amplitude vs frequency) is pretty much everything. I'm not sure it's 99%, but it's up in that ballpark. But to actually achieve such an accurate mapping of one IEM onto another via EQ? I think there'd be more chance of me winning the lottery. I think the best demonstration you could hope for would be John's suggestion (if I understood this right?) of figuring out the EQ using a coupler and measurement software, recording both headphones on that coupler mic, and then playing the recordings back on a common headphone.