I know per the RAW that dual wielding bolters w/ melee attachments is legal, but I would not allow it. For one, it's never been done in the fluff, to my knowledge (DW full bolters, excluding forearm mounted). Secondly, I would think a melee attachment should require two hands to use, otherwise why have a dedicated chainsword at all?
I grant you that it seems a bit off. That is until you look at . . .
Bulging Biceps (114) [Space Marine]: Penalties for firing heavy weapons in Semi-Auto burst and Full Auto without bracing burst are reduced from -30 to +0.
and . . .
Unnatural Strength (136) [Space Marine]: Double Strength Bonus.
I would imagine someone with such raw physical presence could handle it. The chainsaw looses balanced.
Also I would imagine if taken as a signature Wargear sufficient modifications could be made to make it, possibly, work in your mind.
Example: the Space Marine's fist is inserted into guarded hilt and grasped by all five fingers (like grabbing onto a shovel handle surrounded by a metal guard and making a fist). The firing mechanism would require the pull of a trigger, and the chainsword attachment would require the switching on of switch (perhaps by the thumb) and would be quite secured (assuming that the objection is a melee weapon with a pistol grip).
As interesting side note that made me think of the next bit . . . in WWII in the winter and in otherwise cold areas rifles were outfitted with a squeeze trigger mechanism so that soldiers wearing mittens could shoot.
Perhaps the chainword/bolter has a curved handle similar to Count Duku's lightsaber (in Star Wars), only with some sort of squeeze trigger to fire the bolter part of the weapon.
Also one could argue that having a chainsword attachment increases the space threatened by the melee attachment enough to use a bolter with said attachment as a melee weapon (as supported in the rules).