Is one big file as secure as X small files with the same password?
It depends on the possible attack vectors.

Generally password reuse is very dangerous. For instance, you could have less important files in one volume - say that the photos are less important. An attacker could ask you to show him a photo or two. You enter the password and the attacker performs classic shoulder surfing to get to know the password. Now the password is known for the important text files as well. Possibly you would not have entered the password if both sets of files were in one volume. This is one of the many reasons why you should not share passwords of internet sites either.

At the very least you'll have to enter the password twice to unlock all the files, it seems to me. So having two times the same password is less secure than having one whatever you do.



If you want to use one password: use a password manager, preferably on an unconnected device and unlock two highly complex and different passwords with the one used to unlock the database file.