To begin from the conclusion, you will not save the Ltd. Express fees by making reservations online.
-
http://tobu-discountpass.tobutravel.co.jp/client_en/dpass.php#all_nikk...Please note that Ltd. Express KINU trains are not available at Tobu Nikko Station.
Tobu railway timetables for Nikko / Kinugawa travelers are in a pamphlet concerning discount passes, which is available in a PDF file.
-
http://www.tobu.co.jp/foreign/pdf/pamphlet.pdf [Cost on passes and a Ltd. Express service]
Ltd. Express fees of Tobu may depend on trains. A time-zone discount applies regarding some of the trains bound for Kinugawa-onsen which are to depart from Asakusa in the afternoon
and some of the trains bound for Asakusa which are to depart from Kinugawa-onsen in the evening.
This time-zone discount does not apply when you take KINU 103.
The regular weekday price of a Ltd. Express fee for Asakusa -- Shimo-imaichi is 1300 yen. After a discount by 20 percent for pass holders, the price becomes 1040 yen.
So, the outward Ltd. Express cost for the two adults is 2080 yen.
When you take KINU 138, the time-zone discount applies.
The price after the time-zone discount of a Ltd. Express fee for Shimo-imaichi -- Asakusa is 1000 yen. After a further discount by 20 percent for pass holders, the price becomes 800 yen.
So, the homeward Ltd. Express cost for the two adults is 1600 yen.
The price of an All Nikko Pass for an adult is 4400 yen. So, the cost on passes for the two adults is 8800 yen.
As a result, the total cost in your case on All Nikko Passes and a Ltd. Express service for the two adults will be 12480 yen.
[How your passes may pay off]
A regular one-way base fare to travel between Asakusa and Tobu Nikko is 1320 yen.
The regular rail cost for the two adults will be 9880 yen (= 2 x (1320 yen + 1300 yen + 1320 + 1000 yen)).
So, I suppose your All Nikko Passes will pay off if you use them to save other costs by more than 2600 yen for the two persons, i.e. by more than 1300 yen per person.
----
Tobu Nikko Free Pass
-
http://www.japan-guide.com/e/e2358_002.html