Multiarch Classing (5e Variant Rule)
From D&D Wiki
But what if multiclass is no good, but other variant are is be?
(In the explanation below, I use Sorcerer as it is the class I'm most familiar with. However, any class can multiclass into itself using these principles.)
If you want to multiclass into the same class but with a different variant, with this homebrew ruleset, now you can! Firstly, you MUST meet the multiclassing requirements for your own class (For normal classes this is in the Player's Handbook). You likely will already meet this requirement, but it is one because of the difficulty accompanying the change. When you gain your first level in the multiclass of your own class, start keeping two separate counts for your level. When using the separate variants, use the separate level counts to determine the benefits. For the record, the "variants" I refer to are the parts of a class in which you have to pick between two (Or sometimes more) sets of benefits, and often can't change this choice later except with some rare exceptions. A Sorcerer, for example, can only have one Sorcerous Origin, with Draconic Bloodline or Wild Magic. If you were to multiclass this way you could do both. Using this method, if you had 1 level in Draconic Bloodline and 1 level in Wild Magic, you'd gain the benefits of both but at their 1st level in the class. (If you use a class that doesn't give these benefits until later on, like at 3rd level, then your multiclass level with this new archetype would have to be 3 to gain any benefits. This prevents you from having more class features than you would normally have at max level.)
For parts of the class that are not part of the archetype, such as your spellcasting, hit points, hit dice and normal class features, the combined level of both your archetypes determines your level for dealing with these upgrades. This means you're really only partially multiclassing, in case you didn't know that already somehow. This also means that, say you had 5 levels in Draconic Bloodline Sorcerer and 5 levels in Wild Magic Sorcerer, you'd be a 10th level Sorcerer for everything else, meaning you'd have your new Metamagic ability as if you were 10 levels in Sorcerer, and would also have all the same spellslots as if you didn't multiclass at all.
If you multiclass into the same class using this homebrew rule, you can never master all aspects of either of your Class Archetypes, so keep that in mind (Unless you use a homebrew rule that increases the level maximum, which I believe does exist on this wiki). Additionally, if the DM allows this ruleset, they can use a variant in which you can't multiclass normally if you choose to multiclass your Archetype.