{
    "A": [
    {
       "B": 0,
       "C": 1
    }
    ],
    "D": [
    {
       "E": 2,
       "F": 4}
    ]
}

import json
from pathlib import Path
path = Path('newtest.json')
data = json.loads(path.read_text(encoding='utf-8'))
data['A'].append({'B':50,'C':50})
path.write_text(json.dumps(data,sort_keys=True, indent=4, separators=(',', ': ')), encoding='utf-8')

{
    "A": [
    {
        "B": 0,
        "C": 1
    },
    {
        "B": 50,
        "C": 50
    }
    ],
    "D": [
    {
        "E": 2,
        "F": 4
    }
    ]
}