In Parallel should be louder, IE 2 X 8Ohms = 4 Ohms.
Thats what I did in a Bachmann DMU last week, yesterday I did a Cavalex 56 with 2 EM'0' speakers at 4 Ohms wired in series and WOW. What a noise.
https://www.dckits-devideos.co.uk/index.php?route=product/product&path=223_75&product_id=406
Demoing thse this weekend at Doncaster & shortly at Glasgow & Preston