Multiple Values in single slot

I am new on rasa_core famework. I want save both the values 2 and 4 in a single slot and pass the value in action_calculator. Action Calculator will fetch the value 2 and 4 from tracker after that It will perform the addtion of 2 and 4. Currently I am getting only the last number 4. Please help How to resolve this problem ?

In nlu data train

## intent:calculator
- Do the addtion of [2](addtion) and [4](addtion)


## story_calculator
* calculator
 - action_calculator 

In domain.yml

               type: unfeaturized