Firstly, I would refer you to the Dungeon Master's Guide. In one of the appendices (Appendix B, I believe), there is a section on crafting a religious system for your setting. One of the options is dualistic which seems to be what you are describing (strictly speaking it is monotheistic from the description, but generally speaking, both the positive and negative "powers that be" have roughly equal power "put into play" as it were. Compare, for example, The Lord of the Rings).
Secondly, I disagree with your associations of Warlocks (and to a lesser extent Paladins), especially in 5th Edition. Depending on how you craft your lower planes, a Fiendish Pact Warlock may necessarily have a Patron in Satan(if you decree that all such pacts go to the top of the hierarchy), but unless you exclude all other power sources from your game(which does have additional side effects that I will address later), they have other options. Are there no Great Old Ones in your campaign? Are there no Archfey?
Note that in general, there are more factions and powers at play than just the heavenly deities and infernal and abyssal fiends. These factions (even the chaotic ones) tend to be hierarchical, with localized powerful creatures belong to each (You could say that the default cosmology is inherently feudal). If you remove Fey from your mythopoeia, you as a DM are free to do so, but doing so also you behooves you remove the Fey entirely (which has ramifications for Druids, some Rangers, and the seemingly Fey-aligned Paladins of Light) or explain why they are not overwhelmed by the other factions. Likewise Great Old Ones are the heavyweights of the Aberrations. Again, you can remove them from your campaign, but doing so limits player choices and the variety of your world and the adventures that can take place in it (and presumably Celestial Warlocks are right out).
Additionally, Paladins, even in a duopolistic system, do not get their powers from a deity in 5th edition, but rather from their Oaths. So a Paladin of Devotion, may have your singular deity as their power source, but in a dualistic system, a Paladin of Vengeance might easily be empowered by Evil forces.
tl;dr: reducing the deities into a dualistic system is supported and can be fun, but I suggest not making those two sides the only two super-mortal power sources in your game, because it reduces the tools in your toolbox.