The [0] would be incorrect you would need to use Jinja to parse the string apart at the semi-colons, they are not an array, just a string.
Probably:
value_template: '{{ value_json.sumstate.value }}'
Is closer but the state would just be the string. Your have to further process that string by splitting it. See my other post for a different recommendation but it all depends what you want to use the data for.
Still be specific. You want it in a sensor to? Display it? Run automations based on its value?
These are two very different things.
You can easily pass in the state of your generic sensor that gets the string value from the JSON into two other template sensors that grab the values you want. BUT, I would only do that if you want to take action based on their values. It you only want to display them, there are much better/cleaner/less complicated ways.
Shows nearly exactly what you want if you only want two sensors/two values. You just need to augment the Jinja to split the “value” at semi-colons and grab the value you want.