hopey
A short reply to concur with Ken, now that I understand your question better: it’s not that the “extended” channels are all in HD. They are not. They are digital as opposed to analog. So you have one digital box and your other box(es) are analog and cannot decrypt the digital signal. So yes, all of your feeds need to come off the one digital box.
I have a digital cable box. My TV is not HD. I can view every digital channel except the ones which are specifically listed as being HD, because my TV will not display them.
Keep in mind that if you hook up different TVs to your one digital cable box, you will only be able to change the channel from that one box, wherever it is located! So if the box is in the living room but your TV is in the bedroom, you’ll have to figure out how to change the channel from the different room!