religiously ok to do it?

if someones surname is HUSSAIN and after they get married the marital name is ABBAS

is it ok to change the girls surname to Hussain-Abbas

Depends how you view surnames... if it is customary for surnames to denote the name of one of your parents then changing the surname like this would not be appropriate...

e.g. Aisha Hussain = Aisha (daughter of) Hussain

so... Aisha Hussain Abbas = Aisha (daughter of) Hussain (son of) Abbas.. which isn't right...

For this reason many Muslim women retain their maiden names

"Call them by (the names of) their fathers: that is juster in the sight of Allah." (Qur'an 33:5)

i wasnt asking about customs and legends, thanks for your reply.. i was asking if its ok to have both these big names as one name..? a double barrelled name

i dont see why not, but then im no expert.