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I guess that double split guard ring is the guardring which consits of substrate contacts and from the well connected to gnd or vdd - depends n process (P/N substrates...) The reason is to trap carriers before those can cause any harm like triggering on latchup for example.
width - I know about short channel effects but width....
The only think is that during processing you have a mismatch so two neigboring devices can have slightly different W. If you consider W=1um then 10% change has more effect on device performance and matchiing then in case of W=100um