Are 60 seris and 80 seris split rims the same?

No, different offset and different appearance.

The 80 series split has more backspacing than the 60 series split.

I know a few people who use the 60series splits on their 80series because it widens the track.

No. As above.

As Phil said you can run 60 series rims on an 80 resulting in a slightly wider track, might upset the legalities though ;-))
80 series rims won't fit on anything earlier as they foul the front brakes and narrow the track, not something you want to do on 75 series ;-))
A relatively common change is to fit 60 series diff housings to a 75 as it widens the track by about 80mm, then fit 80 series rims which widens it further, increases the stability of a 75 no end. Also solves the problem of the small front diff in a 75