Yep, the cable going to the modem also carries the TV too.
However, you cannot fit a 2nd STB yourself. If you buy a "2nd hand" box to fit yourself it will not work.
Only Virgin Media supplied & installed STBs will work - they must be registered against a valid account, and activated by VM & paired with a smartcard.
If you take out the "2nd box" option from VM, they will do the installation for you & fit a splitter (or run a new feed if it's deemed necessary).
BTW, If you get the new "V+" STB (3 tuners, HDD PVR, HDTV support), & keep your existing STB & have that moved to the other room, the installation is free (using the old STB as a 2nd box costs £5/month while the V+ costs £10/£15 depending on if you have TV size XL or M/L).
http://allyours.virginmedia.com/html...lus/index.html
http://allyours.virginmedia.com/html/dtv/vplus/faq.html