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Orange,
It's a form of back to back ticketing (b2b,or back ticketing) that the airlines frown upon. It's not illegal but might be against your airline's regulations. Generally it's used to avoid minimum stay requirements, where business flyers buy discounted tickets with overlapping travel dates to take advantage of lower prices for fares with Saturday night stay requirements. I think you are probably okay with the way you intend to do it. To be completely safe you could use different airlines for each itinerary.