I tired this out myself today (before having read this post) and I discovered that they don't stack.
The base range for shipyards is 3 tiles.
With either the ideology trait OR the Engineers bonus, it extends to 6 tiles.
Theoretically one can extend shipyard range to a maximum of 9 or 12 tiles (depending on if the bonus is added to the base or to the base plus engineers, though I imagine it would be a multiplier to the base) if one takes both engineers and the ideology.
This however, does not work. When I anchored my shipyard at 7 tiles away from my homeworld and launched a colony ship it launched empty. My homeworld's production also went into the white though it told me that shipyard decay was at 0%.
This seems like a bug to me, but I don't know. Perhaps it is an intentional limitation. If that is the case, I believe the player should be warned before choosing the Engineers trait.
As it stands, the trait is only useful if you forgo the Pragmatic branch that improves starbases, mining and constructors because you will be wasting an ideology point if you do.
The trait is currently only useful for Evil or Benevolent civs if one plays to specialize down those branches alone.
Long story short, either more player feedback is needed regarding the trait and ideology or some tweaking is required.
Thus far the devs have been good about addressing issues so my faith is high. ;3
