Can one aircraft alternate between two routes?

Yes. An aircraft working more than one route flies every trip of the first before starting the next — Delhi, Delhi, Chennai, Chennai. Open it on the Fleet tab and tick Alternate routes under Rotation to take them in turn instead: Delhi, Chennai, Delhi, Chennai.

It flies exactly as much either way; only the order and the times change. Spreading each route through the day puts your hub's arrivals and departures between one another, which gives connecting passengers shorter waits. Because the times move, the change is checked against your slots like a re-time, and new slots are offered at the usual price if it needs them.