If I buy a 2nd hand sky+ box and put it in place of the old multi room box so I have 2 sky+ boxes will I have to pay sky any more in monthly subscription.
Definitely not if you are on a decent package which gives you the SKY+ facilities for free.
And even if you are on one of the cheaper packages which incur a £10/month subscription to activate the SKY+ facilities on your existing box, you mind find that the Multi-room subscription transfers this to the new box for free as well (but I'm not 100% on that)
The £10 multiroom is for a mirror of the first subscription, so you will not have to pay the second extra £10. You will need a second feed of course and to pair the card to the new box.
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And even if you are on one of the cheaper packages which incur a £10/month subscription to activate the SKY+ facilities on your existing box, you mind find that the Multi-room subscription transfers this to the new box for free as well (but I'm not 100% on that)