I know we've discussed this before on the boards, and I believe that the consensus was that by RAW, it truly does say "any weapon" (or some wording to that same effect). So a Heavy Weapon is definitely viable for Slayer-Limbing.
That being said, again by RAW, there is absolutely nothing saying that rules regarding such things as Bracing would be waived. So you would still have to brace, and don't forget that weight is not waived either, so hauling two (or more) heavy weapons around might be hard.
And while there are no RAW rules regarding it, any GM in their right mind would come up with flavourful reasons as to why there would be problems with the setup you suggest, although not necessarily very hard penalties.
One idea that comes to my mind is an Obliterator or Obliterator-like Greater Minion of Chaos, or some kind of corrupted siege/combat-servitor.
Your final sentence doesn't make sense to me, although honestly, it might be due to sleep deprivation. If only one arm is a Slayer Limb, how.. wait, nevermind, you are suggesting having one regular Heavy Weapon and one Slayer Limb Heavy Weapon. I was thinking you'd have two Slayer Limb Heavy Weapons. I need to sleep.
The only caveat I can think of is potential GM fiat and the fact that you need the GM to actively support the idea in order for you to receive a Minion with Slayer Limb specifically, since the Mutant trait is otherwise a random Gift of the Gods. At that point, though, wouldn't you be better off spending two traits to get two Slayer Limbs, instead of the multiple arms?
Looking at traits alone, assuming a Greater Minion of Chaos (7 Traits) and as long as the GM is on board with it, I'd go with something like:
Auto-Stabilised, Slayer Limb (2), Size (5; Hulking), Regeneration (4), Unnatural Toughness (4), Unnatural Strength (4).
I'm envisioning something half-flesh-crafted, a hulking ogryn-like beast with an intelligence somewhere south of Venus, clad in either a skin-tight carapace suit with power-assist or a massive breastplate of writhing steel and spikes, depending on your approach, Both arms flesh-fused to enormous weapons; M41 Multilasers comes to mind.
Then argue with the GM until he accepts two shoulder-mounted MIU Rocket Launchers, too.
Edited by Fgdsfg, 11 July 2014 - 06:36 AM.