As far as I am aware, there is 1 known instance of something that lived on earth that, technically, had 6 wings, and 2 of said wings were actually protowings, so truly it only had 2 pairs of functional wings.
Aditionally, the microraptor probably didn't really flap its leg wings, nor did they splay them out like in many depictions, since such an action was likely impossible without involving dislocating them from the hips, maintaining a position closer to an "x-wing" ship from star wars. That coupled with the debate over the microraptor probably being an exclusive glider, as far as the reconstructions' good gliding ability and weak pectoral muscles show.
These traits, coupled with the factor that no modern vertebrate animals today have more than 2 wings is a good demonstration on how having only 2 wings was most likely selected as more advantageous (regarding birds, it left a pair of limbs free for other activities and meant the bird didn't have to worry about damaging their leg feathers on the ground).
So, by far, we're yet to have actual data on a vertebrate which engaged in active flight with 2 pairs of wings. The fact that no known arthropod species ever truly used 6 wings to fly can hopefully show that redundancy isn't always a good thing (the arthropod species with 6 wings went extinct naturally, as did the 4 winged dinosaurs). The sheer fact that the 3 main vertebrate groups to achieve flight were all tetrapods which only used 2 limbs as wings also shows how an additional pair of wings isn't really an advantage.
So summing up: unless I've missed some critical information, no creature on earth, vertebrate or not, ever used more that 4 wings to fly. That coupled with the problems of turning your wings into legs and how no vertebrate on earth has more than 4 limbs leads me to the following theories about your 6 winged angels:
-1: they're, like the biblic angels, supernatural entities which don't need to bother about things like an efficient body. If they wanted to, they could have a total of 10 limbs (4 human limbs and 6 wings protruding from their backs) and still function normally. They fly however they wish to fly, regardless on whether they feel like flapping their wings while flying or not.
-2: they're a race of arboreal 6 limbed beings which were adapted to climbing and gliding to the extreme, with 6 limbs doted of claws, opposing digits and gliding feathers; which, at some point, developed psionic abilities, meaning they could now lift their own bodies in the air and hover above ground, leaving the trees without worrying about damaging their beautiful feathers. They fly as they normally move near the ground: without using their wings since they're not adapted for active flight, but rather via telekinesis (there's a possibility of some younger seraphim hovering up to great heights just to use their wings to glide until they're closer to the ground, until they tire or as a sport, but that's up to your world) .
Idea number 2 also gives a good model of their home: a place where things like stairs and Elevators probably don't exist, or at least not as in our world, since the inhabitants can simply hover to the level they wish.
Now, how do angels fit in this and how do they relate to the seraphim? Simple: while the seraphim went through the evolutionary path of humanoid psionic microraptors, the angels went through the path of humanoid birds, with their bodies becoming more adapted for active flight, but with lesser development of psionic abilities, as their bodies already demand a large amount of nutrients with a brain closer to ours. The 2 sapient races, like humans and Neanderthals, evolved at similar periods and eventually met, living together, potentially with a clear caste system if the angel race's less developed psionic abilities result in them being seen as lesser. Whether the cherubs went on a path closer to the angels, also having 6 limbs but with legs more humanoid and the 2 upper limbs developing more into wings, or closer to the seraphim, loosing 2 limbs and adapting the remaining ones towards gliding, is up to you.