Instead if getting a 2nd box, why not get a video sender instead. They're about £30 (Aldi often have a good one in), and it's a one-off payment, not a constant £15 a month.
We had a 2nd box in from day one (6yrs ago), but when they changed the fee from £5 a month to £15 a month I got it taken out because I thought it was extortionate. You're already paying your package subs, all you're getting is a 2nd outlet, no extra channels.
To be honest, we don't even use the video sender that much now, I'm normally on the PC up until I go to bed anyway nowadays, but it is a more cost effective option than a 2nd box at current prices.