It appears that it will work to wire the lights in parallel, and I should get nearly full output from both when used together.
Response from SON regarding parallel wiring of the E3 and Edelux lamps: (my questions in blue)


Dear Mr. Clifton,

you wrote us:
>I have a SON28 hub and would like to use a SON Edelux lamp as my primary
>light, and a Supernova E3 as an optional lamp for night time mountain
>descents.
The E3 come in different variants. All of them have less brightness in the
center but an overall much wider light distribution. For off-road use the
E3 might be a good choice. All the following is only valid for the
single-LED E3, not the triple LED which I don't have at hand right now.

>Does the SON28 hub produce enough current to operate both lamps wired in
>parallel so that I may use the following combinations:
>a) SON Edelux lamp only
>b) Supernova E3 lamp only
>c) Both lamps together

In case c both lights operate at reduced power. If you aim both lights to
the same spot on a wall, you will have 100% brightness in case a, 70% in
case b and about 90% in case c. I tested this at 15 and 30 kmph. The
advantage of case c is the better illumination of the side areas of the
road/trail.

Clarification: The Edelux is being used as the 100% measure. Therefore, the E3 appears 70% as bright as the Edelux due to its wide beam pattern, and the combination of the two is 90% of the brightness of the Edelux alone, but gives a fuller coverage; which is really the effect I was hoping for.

>I am not concerned if the parallel circuit will only have enough current
>at higher speeds, since the dual lamp operation will be used specifically
>for high-speed night time descending.

It works (with the components I tested here) at any speed.

Gruß
Andreas Oehler

P.S. I presume you are located in northern america. So I take the liberty
of sending a copy of this email to our american distributor Peter White,
who might also be interested in this kind of application.