Can I copy a file to two drives at the same time from a single read? (Copy one-to-two in single operation, preferably in rsync)

Putting together info from several sources, there are a few options.

The conclusion is that only with command parallel you might get what you want, see below.

Important notes:

1. I have made tests with cp for copying. You should also consider speedups (or down!) obtained with rsync vs cp or other alternative commands, combined with parallel.
2. I have tested copying only one file. Results might change if copying many files (e.g., combining a few large files, as you need, with many other small files and subdirectories).

time <option #N, copying to one target>
time <option #N, copying to two targets>

to get a comparison, with a file of 1.2Gb. Moreover, in some cases I tested two or three times the same command, to assess the dispersion in the results. I did not compute averages and standard deviations, but the results are obvious.

This is what I got under the testing conditions specified above, with brief comments. I have concatenated in a single row the results of multiple tests, whenever available.

The base case:

$ time cp -p source/file1 target1/

real    0m0,846s    0m0,680s    0m0,659s
user    0m0,000s    0m0,001s    0m0,016s
sys     0m0,777s    0m0,662s    0m0,643s

The copying options:

1. Option parallel

   $ parallel cp -p source/file1 ::: target1/
   real    0m0,745s    0m0,740s
   user    0m0,121s    0m0,108s
   sys     0m0,609s    0m0,619s
   
   $ parallel cp -p source/file1 ::: target1/ target2/
   real    0m0,794s    0m0,860s
   user    0m0,116s    0m0,134s
   sys     0m1,300s    0m1,380s
   

2. Option tee (appending > /dev/null to avoid output to stdout)

   $ tee target1/file1 < source/file1 > /dev/null
   real    0m0,874s    0m1,040s    0m1,028s
   user    0m0,160s    0m0,172s    0m0,137s
   sys     0m0,714s    0m0,868s    0m0,887s
   
   $ tee target1/file1 target2/file1 < source/file1 > /dev/null
   real    0m1,802s    0m1,680s    0m1,833s
   user    0m0,136s    0m0,212s    0m0,197s
   sys     0m1,642s    0m1,468s    0m1,619s
   

   Copying to two targets roughly doubles the time for one target, which is slightly larger than the time for the base case.

3. Option xargs

   $ echo target1 | xargs -n 1 cp -p source/file1
   real    0m0,666s
   user    0m0,021s
   sys     0m0,646s
   
   $ echo target1 target2 | xargs -n 1 cp -p source/file1
   real    0m1,197s
   user    0m0,018s
   sys     0m1,173s
   

   Copying to two targets roughly doubles the time for one target, which is similar to the time for the base case.

4. Option find

   $ find target1 -exec cp -p source/file1 {} \;
   real    0m2,167s
   user    0m0,017s
   sys     0m1,627s
   
   $ find target1 target2 -exec cp -p source/file1 {} \;
   real    0m3,905s
   user    0m0,020s
   sys     0m3,185s
   

   Copying to two targets roughly doubles the time for one target, which is much larger than the time for the base case... a clear loser.

Sources for "multiple copying":

1. https://www.cyberciti.biz/faq/linux-unix-copy-a-file-to-multiple-directories-using-cp-command/
2. How to copy a file to multiple folders using the command line?
3. https://stackoverflow.com/questions/195655/how-to-copy-a-file-to-multiple-directories-using-the-gnu-cp-command

Sources for performance cp vs. rsync:

1. https://unix.stackexchange.com/questions/91382/rsync-is-very-slow-factor-8-to-10-compared-to-cp-on-copying-files-from-nfs-sha
2. https://lwn.net/Articles/400489/
3. https://superuser.com/questions/1170636/why-is-there-a-write-speed-difference-between-dd-cp-rsync-and-macos-finder-to
4. What's the difference between ` cp ` and ` rsync `?