I run my second keg as carbonated water and add the syrups when water is dispensed. This would also stop the issue of the keg having a soda flavour stuck in the lines.
Not sure about the line length I'm afraid. Compromise and go with 25ft?
Brewing: Brewer's Choice Black Country Mild w/ DME
Keg 1: Muntons Connoisseur's Yorkshire Bitter
Keg 2: Carbonated water
Planned (Beer): Twisted Stout (BIAB)
Planned (Soda Syrups): Orange, Cream Soda, Lemon, Lime, Cherry.
"If I could have a beer with Jesus, heaven knows I'd sip it nice and slow..." - Thomas Rhett: Beer with Jesus.