Write a Python function merge$\_$dictionaries that takes two dictionaries and returns a new dictionary containing all the keys from both dictionaries.$$

If a key exists in both dictionaries, the value in the new dictionary should be a list containing both values.

Test: merge$\_$dictionaries$(\{\text{'}$a$\text{'}$: $1,\text{'}$b$\text{'}$: $2\},\{\text{'}$b$\text{'}$: $3,\text{'}$c$\text{'}$: $4\})$ should return $\{\text{'}$a$\text{'}$: $1,\text{'}$b$\text{'}$: $[2,3],\text{'}$c$\text{'}$: $4\}.$