I had Sky+ Multiroom, a box downstairs and one upstairs.

I upgraded to SkyHD downstairs and cancelled the multiroom, kept the Sky+ box upstairs and it now works as a free sat box. So yes, it's possibly to have one HD box and one normal box if that's what you meant?

AFAIK, you'd have to be on Sky+ for the nonHD box as the HD signal requires the + installation and cabling. So if you're on the old fashioned Sky, you'll probably get stung for 2 times the installation cost? Ring them and ask them.