Is it still possible to SLI GPU's of the same generation?

I remember some 5 years ago it was possible to SLI GPU's of the same generation, can that still be done? For example, an SLI install of a GTX 780 and a GTX 780Ti.

I remember you could set crossfire on a different cards from the same gen but not sli. For sli you need matching product ID regardless what is brand, clock etc.
There were some exceptions but not far from the rule like GTX260 192 could be set in sli with GTX260 216 or 8800GTS 320MB could work with 640MB. Still these were about the same chips. I'm not 100% sure if you can make GTX780 run with 780Ti in SLI but somehow I doubt because of too many changes in GPU itself.

Can you flash bios to match then SLI?

If cards are on different chips then it won't help in anything and driver will see them under different ID. Flashing the same BIOS probably won't even work if shader count or memory capacity is different.
On the same gpu it doesn't matter if one card has higher clock or is different brand or even series like reference and some MSI Lightning or ASUS Matrix. All what matters is the same GPU type.