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I can try my best! So first you need a way to save the history of text. I do this by checking which character was open last and save the text history to a variable.
(replace character 1 and 2 with whatever your character's names are and make sure you have defined each of their nvl names)

label history_save:

    if open_text == "Character 1":
        $ character1_history = nvl_list #base renpy variable that already exists
    elif open_text == "Character 2":
        $ character2_history = nvl_list

    return

Then whenever a text event is called it first updates which character is texting and loads the character's text history. Then after they finish texting it saves.

label Character1_Text1:
    $ nvl_list = character1_history
    $ open_text = "Character 1"#this variable can also be used to give each character different visuals on their messaging screen with if statements
    nvl_narrator "Character 1 added to contacts"
    c1_nvl "Hello. It's me."

    call history_save

    return

That should be all you need for basic texting with multiple characters but if you are like me and want to be able to check back whenever at previous texts you need another way to load their texts. All you need is a contacts screen with buttons for each character(there are plenty of tutorials on how to make gui online). When the player clicks on a button, hide the contacts screen and call the label associated with that character.

label character1_history_load:
    $ nvl_list = character1_history
    $ open_text = "Character 1"
    nvl_narrator "" #just an empty text so the nvl screen shows. I know it's weird but it was the only way I could get it to work.
    $ nvl_list.pop() #removes the empty text
    show screen ContactsScreen #to go back to the previous screen where you have all your characters contacts listed
    return

There are probably more optimized ways to do this but I found this is what worked for me and my game. I also have time based events in place and this worked very well with that!