Multiple switched circuits in single exposed raceway with T-style conduit body to branch them
The discussion in the comments revealed that the "planned addition" as described is perfectly fine but that the only detail that really needed additional confirmation was the conduit fill calculations, specifically regarding derating.

Since I'm using a T-style conduit body to branch a raceway for two fixtures there is not room to have a code-compliant splice between the common junction box and the fixtures themselves. This then means that separate neutrals need to be pulled and that there will therefore be more than 3 conductors in the shared part of the raceway. I had initially read that derating didn't come into effect for small wires (#14 and #12) until a much higher number of conductors are involved, but it was a good exercise to truly confirm this.

The NEC requires derating for 3-or-more conductors in one raceway. My initial impression for #14 THHN was that it's rated to only 15 amps, which would then mean that any derating requirements could be an issue for this 15 amp branch circuit. Looking deeper tough it turns out that 15 amps is only the required overcurrnet protection for circuits with #14 wire (from NEC from 240.4(D)(4)) and that #14 THHN is actually rated slightly above 15 amps at low temperatures. In my case I needed to derate 80% (for 4-6 conductors), and for #14 THHN at low temps this worked out to 80% of 25 amps = 20 amps, which still appears quite fine for the existing 15 amp branch circuit that I'm working with. This handy online tool, and of course the appropriate tables from the NEC (from 310.15(B)(16)), confirmed this.

So it looks like this planned addition is fine as stated assuming a wire gauge &lt;= #14.