Welcome onboard
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As Wizard mentions, it depends on the airline policy and the restrictions imposed by their baggage handlers regarding the maximum bag size they will accept. You won't be permitted to take a single bag of 75kg, I doubt anyone could lift that! As Saga are a charter airline, they don't publicise their baggage regulations as they're often set by the chartering company. Who did you book with? I would ask whoever you booked with what their regulations are. They will often allow the pooling of luggage allowance, but you need to check with them. The maximum bag size will either be 23kg or 30kg, depending on the handling agent. I would agree with Wizard, if you get no joy, stick with 15kg per bag.
Darren