I do not if that is against the rules though, so please do correct me

Its called cheating yourself.

It is not within the rules to play yourself. Aside from that, seems extremely illogical...

Can I have multiple accounts?

YES, Chess.com now allows each member to have TWO accounts, with approval from support. Without support approval noted on the account, this is still considered cheating. If it's just a new username you are after, please see here. 

There are many reasons you may want a second account. Some people prefer to test out new openings on an account where they don't have to worry about their rating, or you may want to keep certain games private. Or you may want an account just to play on when it's late at night and you're tired. Or any number of reasons! But there are also things you should NOT do on your second account. 

If you have two accounts, you may NOT:

• Play the accounts against each other, even in unrated games
• Play in the same vote chess game with both accounts
• Play in the same 4 player chess, or other variant game with both accounts
• Play in the same tournament with both accounts
• Comment with both accounts in the same forum thread or discussion
• Use the account as a 'speed run' account

That's actually really cool.

A casual or messy account is a good idea if you want to protect your 'serious' accounts ratings.

I see nothing wrong with having both a serious account and a casual one.

Right, but it seems like the topic is about actually playing yourself? Because otherwise how can the main actually be higher (as opposed to just the alt being lower?)
And btw, I reported someone who did that a while ago, because this person was so much underplaying their supposed strength that I took a look and saw that all their rated games were obviously set-up. It was a sad account, you look at their page and it is a bunch of losses in unrated games. This account was eventually closed for abuse

So, I played against my mom in my alt + my main account in both daily and rapid a few times using queen hayward checks. She is a beginner at chess and doesn't know how to check, openings attacks, etc. Before anyone bans me for using an alt account to get elo, its just thats the only alt account I have for my mom and I to play together online since we don't have an real life chess board. She is using my alt to play against me and im just so clever to trick her in real life to checkmate her lol. This is my alt account which im using to type this comment up.

I have an alt to play games but it is supposed to be secret so nobody can search up my games for competitions. the rating is higher but i created 2 years ago so i have improved. so now i only use my alt to play games, while i use this to chat and play variants instead

Yes.. right... my "casual" account... You'll never know when I get serious... 
Ehh. The ratings need to be disabled for practice and casual play, or at least let the players set preference thresholds for their opponents. Depending on your rating range elo and the positions/openings you're practicing, it can be difficult to find the right opponents to help you optimize your game.

Just so I understand, all of the GM's like Hikaru etc. with multiple speedrun accounts are breaking chess.com TOS?

I think they reach out to chess.com beforehand. Also, unlikely chess.com will ban someone like hikaru unless he does obvious cheating or something due to his influence and everything