Conditionally setting one variable's value based on another

See Mixin Guards and Ruleset Guards (aka "CSS Guards") for when usage examples. Since you need to define a variable you'll have to use mixin-based condition (rulesets do not expose their variables to an outer scope). E.g.:

.-();
.-() when (@side = right) {
    @sideOpposite: left;
}
.-() when (@side = left) {
    @sideOpposite: right;
}

(Use any suitable mixin name instead of .-, e.g. .define-opposite-side).

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And with Argument Pattern Matching it can be further simplified just to:

.-(@side); 
.-(left)  {@sideOpposite: right}
.-(right) {@sideOpposite: left}