You can get the same yield from 1 plant or 2 plants in the same size space if you utilize the full space. Your just going to need to veg longer and train if you go with one plant in that space. If you choose to go with 2 plants, then you cut your veg time significantly due to cabinet size restrictions and you will get to a 12/12 light schedule quicker. Flipping to 12/12 when you are 1/3 of your max height is a good rule of thumb to go with since most plants stretch an average of 2-3 times their height in the first 2-3 weeks of flower.
Example:
Your max plant size you can accomodate in your box is 30" so you will want to flip from veg to flower when your plant is 10" tall.