Thank you so much for the super informative and quick responses - and apologies to those I have upset by asking!

I will definitely just book my standby flights based on the loads super lastminute.com in future!

And have never actually booked both at once but was just curious if it was really a no-no given the system would not only allow you to but arguably kind of invites you to do it given that we can't get partial refunds (although I do take the point about the baggage allowance - maybe if they threw in lounge too it'd be a bit more palatable altogether )

Thanks again so much all!