I'll enumerate the exact values for simplicity. =)
Single Disease loses very little on AoE but loses a lot now on single targets because of the ridiculous threat on IT, combined with the same marginal losses from BS, and possibly whatever glyph you could've used in place of HB.
- Ramp up much faster but to a lower cruising threat on single targets, *very* minimal loss on multiple targets (~4-6% from losing Blood Plague).
- On single targets you lose 12.5% OB dmg, 25% BS dmg, and the dmg from PS+Blood Plague, and the dmg/threat from IT.
- Gain one more OB in every 2 rune sets (if you OB on FU pairs that is a 33% increase in OBs fired), which in turn can produce more Rime procs.
- Here Glyph of HB enables this by maintaining your disease without having to use IT, though this is *far* less appealing now than when the guide was originally written.
Double Disease by contrast means you get harder hitting OB/BS, and more disease damage. That can really add up on the single targets.
To answer your question from there, you *can* use the Glyph of HB with a double-disease method, but it is rather less important, and with the threat value of IT now it is really just an insurance policy to be sure that your Frost Fever is always up. You'll almost certainly get more raw threat value from most any other glyph.