In terms of practicalities, you'll need an existing co-ax connection point in the room you want the additional box installed. If you already have this, then additional boxes should be a self-install QuickStart process. If not, you'll need a manned install to run the internal cabling and this may be chargeable.
I don't believe there's any documented cost for additional boxes, but it's generally around the £7/month mark to replicate your existing package on it. You can order it by calling VM on 150 (from your VM phone) or 0345 4541111 (from any other phone)
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Thanks for your post and welcome to the forums. It's really great having you on board with us in the Community.
As mentioned by japitts, to enquire about an additional box you will need to speak to the team on 0345 454 1111. This is so the team can see what type of install you will need. They can also look at your package to see how much the additional box will be. There may be a promotion on of some sort that includes a second box you see.