Hm, tried that in my initial tests, but my other sigils make it harder to test for. After attempting this again, I think I've found my problem. It looks like it isn't 'One Weak Blood shard per sigil', it's 'One weak blood shard for one sigil, per armour'. Meaning you can only have a single sigil in a single piece of armour regardless of how many weak blood shards you socket into it. An important distinction, as I now need to spend another hour or so right clicking an orb to get enough LP to unbind my armour again. xD Welp, lesson learned. I guess I need demon shards if I want 2 sigils per armour, even if I have open slots. Does seem odd to me though since we have so many open slots with which to play with, but nothing we can do with them under that limitation.
For that matter, did I miss something on adding LP to my network? Tier 5 altar, but right clicking an orb takes quite a while to get you anywhere meaningful, and at 25k per unbinding ritual, I really feel like I'm missing something important here!