In the first case, you are specifying two files as copy target, both residing on
remote.net. This is clearly ambiguous, as the program cannot determine where to copy the file.
Consider the following, working example specifying two source files and one destination (directory) using the same format:
scp 'firstname.lastname@example.org:file1 file2' .
This will copy both
example.org to the local directory.
scp program connects to the remote host and starts another instance of of
scp with a parameter determining whether you're transferring files from it (
-f), or to it (
-t). You can see this when you add the argument
-v for verbose output. The source/destination parameter you enter (
host:file) is split and the file names part simply appended as additional parameters specifying the files to use.
When using it as source:
debug1: Sending command: scp -v -f -- file1 file2
This works without problem, it transfers multiple source files over the network.
When using it as sink:
debug1: Sending command: scp -v -t -- file1 file2
This fails, as you're intending to write to multiple files. Now the way around this is to escape the file name again (once for the local shell, once for the remote call):
$ scp foo 'email@example.com:foo\ bar'
debug1: Sending command: scp -v -t -- foo\\ bar
scp foo 'firstname.lastname@example.org:"foo bar"' works just as well.