I’ve read various things about not using return legs being cheaper with airlines sometimes. I’ve found a similar thing with Le Shuttle, the train from Calais to Folkestone.

I am interested in buying two return tickets with Le Shuttle on two different weekends. Would not using the return leg void the second ticket or have any other possible consequences I should consider?

As you can see on https://www.eurotunnel.com/uk/tickets/ a overnight return ticket is cheaper than a single or long stay ticket.

The website says just the following:

+£30 Day trip & overnight – Ticket must be purchased as part of a 2 day return. Return must be completed by midnight (local time) on the second calendar day.