Currently I have a SkyHD box downstairs, I wish to install one upstairs aswell and have it on Sky MultiRoom, I plan on buying the box from ebay, will I have problems getting them to activate it solely as a second box?
Probably. The Sky operators are pretty clueless when it comes to anything non-standard. You'll just have to persevere. Make sure both Digiboxes are connected to the same phone line. Order the second card. Phone up and get them to activate it.