From reading http://www.widescreenmuseum.com/sound/fantasound1.htm I think there where 3 independent sound channels.
Each channel was directed towards a speaker (L C R Ls Rs) according to a control track. Basically they did the mix on playback.

The 3 original channels don’t correspond to any specific speakers. Channel 1 could be directed to L at one point, and later on to R. So you absolutely need the control track to recreate it.

In theory Dolby Surround can reproduce the original mix perfectly.
Modern signal processing gives you quite good separation, even with complex sounds.