Joseph and Joshua are too similar - both start with Jos, both begin with a J and both are 6 letters long.
Their post will be addressed to Mr J Surname. They already share a birthday and surname, such similar first names could cause admin errors.
I would swap one out for a similar name e.g. David, Thomas, Ethan, Luke, Isaac, Matthew.
If they don't both need to be biblical then maybe something like Henry, Edward, William or George.
I think it's nice to give twins complimentary names i.e. with a similar feel, but these are too matchy IMO. Joseph and Matthew for example are both biblical and classic names and compliment each other nicely, but are different enough not to be twee.