from telethon import TelegramClient, sync, events, utils
from telethon.tl.custom import Button

# Вставляем api_id и api_hash
api_id1 = 00111111
api_hash1 = '0011111111111111111111'
client1 = TelegramClient('78901234567', api_id1, api_hash1)


api_id2 = 00222222
api_hash2 = '00222222222222222222'
client2 = TelegramClient('89012345678', api_id2, api_hash2)

@client1.on(events.NewMessage)
@client2.on(events.NewMessage)
async def handler(event):
 
    client =  event.client
    msg = event.message
    # тут код для обработки события
client1.start()
client2.start()

client1.run_until_disconnected()
client2.run_until_disconnected()