#1
I began the finishing on my strat-esque build and started with low gloss Tung oil because I wanted the neck to feel buttery smooth. After applying the first coat I'm worried that it will come out as a very dull finish and I really liked how it looked when first applying it.

Now that I've put a coat of the low on, can I go over it with a high-gloss tung oil product (the same manufacturer makes one,) or will the low-gloss have some sheen to it after a few coats are built up like some of the rub-on varnishes?
#2
Yes, and yes. Tung oil can be applied over itself easily. Low gloss will also build a little bit of a sheen, mostly due to the eventual complete sealing of the wood, and when you use Scotchbrite or a really fine grit of paper on it, you will be essentially polishing it. I would start by mixing a bit of high gloss with low gloss. Since they are the same manufacturer, it should be no problem. Smooth it when hard, then decide if you have to change the ratio a bit. It will build, and fine Scotchbrite will bring out a bit of even shine. Trust me though, you will like the feel of a bit less gloss.
Various Strats
Polytone Mini Brute
Koch Studiotone XL
Swart STR Tweed
Quilter 101 Reverb and Mini
1958 National lap steel
Eastman El Rey 1