Okay, I waited for someone to respond... no takers?
Here is my opinion:
Two batteries...... go with the 08100w (10amps) or OKR-T/6 (6amps) or T/10 (10amps). Note: two batteries in series.
The 04050c is limited to 2.4 amps max. For me, it's sufficient for all my needs and it is ideal if using a Single battery and want a small mod.
If you decide on two batteries, batteries go in parallel.
I have a desk full of various DC-DC converter mods. I like them all. I don't have a two battery 04050c. IMHO, if you are going to use two batts for a 04050c, go instead with the converters in the above first paragraph.
Touch switches... many have had problems. I think they are great but wouldn't put one in a mod I was giving to someone.
What do you mean "selector" switch to change batteries?