Haribol!

I don't think there is an explicit standard, but I think the way Gaura Nitai are worshipped around the world in ISKCON, both are worshipped as equals (ie: non-different as you state). For Lord Nityananda's appearance day, both Gaura & Nitai are bathed with abhisheka...and likewise for Gaura Purnima. So I think your feeling of offering both Gaura & Nitai garlands on their respective appearance days is more in keeping with this way of worship. As far as a standard, I don't think it's explicitly stated...

Other things that our temple follows, is that the large and small (utsava) deities are offered the same things...for chandan yatra, if the large deities (Radha Gopivallabha) get chandan, then the small Radha Krsna deities are also offered chandan, as They are non-different from each other, and the smaller deities represent the large ones on festivals and special occasions...

Hope this helps. Hare Krsna!

SriRKd