How to check in both British and American English as well as using a custom wordlist

Hi would like to check my documents both in American and British
English and I think that can be do with these flags

--master=en_US --extra-dicts=en_GB

However, I also have a custom list of word that I would like to add to
a new dictionary. I think I was able to create a new dictionary with
this line of command

aspell --lang=en --dont-validate-words --dont-validate-affixes create
master ./new_dict < wordlist

where wordlist is a file that contains the list (separate by white
spaces) of words that I would like to add

But now I don't understand how I can make sure Aspell uses en_US,
en_GB and well as the new dictionary that I've created, new_dict.

My attempt was to create a multi dictionary file (dicts.multi) that contains

add en_US
add en_GB

Then I did

cat my_file.txt | aspell -a --master=./dicts.multi --extra-dicts=./new_dict

but somehow the words that I've added in the new dictionary are not
recognized as I get spelling corrections for them. What am I doing
wrong?