That won't make any difference, the higher wattage is to accommodate the tube lengths.Alright. What if I use slightly higher wattage and hang it right from the top? 12" above substrate approx?
If I was you I would stick to just one fluorescent tube. You can put different length tubes in the same holders.
If you get that double one you want, you should still be able to just use one I think.
But its getting untidy with extra wires that you don't need.
Have you already ordered that double tube holder?
A juvenile redfoot doesn't want it too bright.
1 tube approx half the length of you enclosure will be just right.