[POLL ENDS IN FIVE DAYS]
As before, this is an attempt to summarize the majority opinion/interpretation based upon what we have from the Codex and BGB as they currently stand.
Once the poll is over, this will be posted in a web page on Warpshadow.com.
And now, I'll duck for cover!
Ranged Symbiotes 
“A creature with two or more ranged bio-weapons may only choose to fire one of them per Shooting phase unless it is a Monstrous Creature, in which case two weapons may be fired. A Tyranid that carries two of the same ranged weapon symbiotes counts them as twin-linked.” [CT30]
Q: Given the above, how does a TMC armed with 2 Venom Cannon, (which cost twice as much as the full cost of one Venom Cannon which fires at assault X) fire?
WA: The simplest and most conservative answer is - a twin-linked Venom Cannon with assault X shots.
Alternate answers: A) Fire each Venom Cannon once at assault X; B) Fire both Venom Cannons counting them both as twin-linked assault X C) something else
Q: Given the above, how does a TMC armed with two twin-linked Devourers (total of 4 Devourers modelled) fire? Double twin-linking? Does the second pair have no effect?
WA: A tough question, but let's look past the Codex. First, "no single dice can be re-rolled more than once regardless of the source of the re-roll" [BGB, 5]; which should rule out "double" twin-linking, or re-rolling misses twice. If the codex had a special rule for double twin-linking, we might worry more. Second, units do exist in the game that have multiple pairs of twin-linked weapons and they fire them individually, and it would be odd to gain nothing at all from buying a second set of Devourers. So, we are going with - Fire each twin-linked Devourer once at assault 2X, re-rolling misses.
Close Combat Biomorphs 
Q: If my TMC has Crushing Claws and a Ranged Bio-Weapon, how many shots does it fire?
A: That is covered in the Crushing Claws entry [CT31].
Q: Do Rending Claws affect Bio-Plasma?
A: The fact is, as written, one can make the argument that RC affects BP - "Tyranids with Rending Claws gain the Rending ability for all their close combat attacks" and BP allows one to "make a single extra close combat attack"[CT31]. However, BP is specifically a big blob of spit that occurs out of sequence with the other attacks, as opposed to just another variety of tooth and claw attack[CT31]. The Codex does point out that Monstrous Creature and Implant Attack do not affect BP, further reinforcing the notion of a separate, special attack unto itself. For now, we are going to assume that RC does not affect BP, although we admit that it is more of a "common sense" argument and we wish we had stronger rules wording to back it up.
Q: Does the Lash Whip & Bonesword combo grant an extra attack in close combat for a second weapon?
A: No. "[Scything Talons] is the only way that Tyranids gain extra attacks in close combat for multiple weapons" [CT31, last sentence under Scything Talons].