There are a few ifs and buts.
Main idea is that adding two lights is always brighter than just the one brightest light.
The camera automation sets shutter speed and/or aperture for the ambient light, and then automatic flash must work into that situation.
The Nikon flash metering system has two flash modes, called TTL and TTL BL. The default is TTL BL. When the menu simply says TTL, the default is still TTL BL.
1a. Case of one TTL BL flash in bright sunlight. In A or P modes, the ambient sets the camera exposure. The TTL BL flash power is automatically reduced, perhaps -2 EV, to be more appropriate. The -2 EV does not show in the menus, but it does, and the Exif will report it. Because otherwise, proper ambient exposure and "proper" flash exposure is 2x proper, or one stop overexposed by definition (two lights added). So TTL BL tries to tend to that, and reduces the flash power. This is called Balanced Flash. Technically, it means reduced flash.
1b. Case of two TTL BL flashes - two flashes still add. Sum depends on how they are aimed, into same one area, or into two different areas. Camera does not know, so IMO, the Nikon flash metering system (the Commander for two TTL flashes) routinely underexposes one stop to account for this. We can add it back as seen needed.
I don't know what your non-Nikon Commander does, but sounds like the same thing. If your flashes optical mode is compatible with the D750 commander, it would be an interesting comparison. Your experience indicates you need -1 EV overall. My own experience with Nikon Commander is that more often I need +1 EV overall (with TTL flash).
2. But also.. default TTL BL is "Automatic flash". Regardless of what you do, it does things depending on what it deems necessary. This means it can ignore or modify or override your own flash compensation efforts. Your efforts do NOT have zero effect, but sometimes they have minor effect (on TTL BL, which is default). So you are trying to control automatic flash which has its own mind about what needs to be done. When fighting automation, automation usually wins because it's too dumb to relent.
A few individual flash models have their own menu to specify TTL or TTL BL flash mode, but this is disappearing. I sure miss it, seems necessary to me. Any Nikon iTTL we see now is TTL BL default. The Nikon flash metering system default mode is TTL BL. However, TTL BL is automatic compensation, and TTL mode (or Manual flash) is superior if you instead plan to control your own flash. Actual TTL flash or manual flash gets rid of the hidden other force trying to manipulate the flash.
Actual TTL flash (instead of TTL BL) does what the camera flash metering says should be necessary, regardless of ambient or any other light. TTL BL does things based on assumptions about other lights. This is the camera metering, not the actual flash, which just responds to power commands.
Setting camera metering mode to Spot Metering will force TTL metering mode instead of TTL BL mode.
The flash does NOT do Spot metering then, it has its own metering system more like Center Metering (regardless). Ambient of course will respond to Spot Metering (very important outdoors, but indoors, ambient is normally too dim to much matter what it does, so indoor ambient is normally much underexposed if with flash indoors - excepting high ISO of course, when flash becomes fill to ambient). But Spot metering will change TTL BL to be TTL mode, which changes how compensation works. Your compensation efforts will be more obeyed. This TTL / TTL BL mode change is not reported in the menus, but Exif will show it afterwards.
So two choices... get used to what the TTL BL system does, and learn to live with it. Do what you see you need to do. Once you know what to expect, this seems no big deal. Sounds like you get your desired results.
Or you can use Spot metering to force TTL mode, and then numerical compensation will be more what you expect. TTL compensation is relative to the metered TTL exposure (Not relative to ambient metering).
But note that outdoors in bright light, Spot metering has very large effect on the ambient, whole different game needing understanding of Spot metering.
Three choices actually, of course manual flash allows total control.