A condition with multiple commands

Consider set -e.

If this is in a script, then you'll probably want to remove the if...then entirely and just put set -e at the top of the script. This causes the script to stop immediately if any command fails. When any of those rsync commands fail, they'll display error messages explaining the failure, so you probably don't need to write your own message at all.

If you want to show the commands that are run before running them, use set -x. You can combine the two by writing set -ex.

With &&, you don't need \.

If you do decide to write something along the lines of what you've shown, then as Ronny Blomme says you can separate the commands with &&. Note that you don't need to run the commands in subshells, i.e., you don't need to enclose them in parentheses.

If you write an && operator at the end of a line, then you don't need to write \ to specify line continuation. A command can't end in && (logical AND) or || (logical OR) so the shell will continue parsing onto the next line.

if
    rsync "$folder/coopshop/prod" "$folder2/coopshop" &&
    rsync "$folder/coopweb/prod" "$folder2/coopweb" &&
    rsync "$folder/unihobbyshop/shop" "$folder2/unihobbyshop" &&
    rsync "$folder/unnihobbyweb/web" "$folder2/unihobbyweb"
then
    echo 'OK! rsync commands completed successfully!'
else
    echo 'Error! some rsync command failed!'
fi

You don't have to put && at the end of a line. You can write multiple commands on the same line separated by && if you like.

Note that the final command being tested doesn't end with && because && separates the commands. If any command fails, the others will not run.

Running all the commands even if some of them fail

You may instead want to run all the rsync commands even if some of them fail, then test if any of them failed and operate accordingly. For this, you can trap ERR. This is not an actual signal, but the ERR signal_spec in bash catches commands that fail for any reason, including a crash (and including SIGINT). See pLumo's answer (but omit set -e to keep going) and this post by William Pursell; for more general information, see the output of help trap.

The way I like to do it (with commands like yours plugged in) is:

failed=
trap 'failed=yes' ERR

rsync "$folder/coopshop/prod" "$folder2/coopshop"
rsync "$folder/coopweb/prod" "$folder2/coopweb"
rsync "$folder/unihobbyshop/shop" "$folder2/unihobbyshop"
rsync "$folder/unnihobbyweb/web" "$folder2/unihobbyweb"

if [ -z "$failed" ]; then
    echo 'OK! rsync commands completed successfully!'
else
    echo 'Error! some rsync command failed!'
fi

About the other changes I've made... and what if command really tests.

Your arguments to rsync use parameter expansion so I've taken the liberty of double-quoting them. Otherwise, word splitting and globbing will be performed. From context it appears you don't want those additional expansions. (Usually you don't.)

Note that, contrary to the message your script prints, you are not actually testing if any rsync command crashed. You are instead testing if any rsync command failed. Crashing is abnormal program termination, where a process is terminated by a signal, typically due to a bug or a condition that cannot be reasonably handled. This is far rarer than normal program termination, which may itself occur with an exit status indicating success (zero) or failure (some other number). If rsync does crash, the code above will catch that too. But most of what it's catching, in practice, is normal program termination with an exit status indicating failure.

For a script like yours, I would not use if/else, but enable exit on error (set -e) and trapping.

Something like this:

#!/bin/bash

set -euo pipefail
trap 'echo "Error! rsync crashed!"' ERR INT

rsync $folder/coopshop/prod $folder2/coopshop
rsync $folder/coopweb/prod $folder2/coopweb
rsync $folder/unihobbyshop/shop $folder2/unihobbyshop
rsync $folder/unnihobbyweb/web $folder2/unihobbyweb

echo "OK! rsync done!"

try this syntax:

#!/bin/bash
mkdir dir1 dir2
echo "test1" > dir1/file1
echo "test2" > dir1/file2

if (rsync dir1/file1 dir2) && (rsync dir1/file2 dir2) 
then
  echo "OK! rsync done!"
else
  echo "Error! rsync crashed!"
fi

I tried this.

commands=$(
mkdir 1
mkdir 2
)

if($commands); then
  echo 'Ok.'
else
  echo 'False.'
fi

It works. Problem is when I add for example ls -la to commands, it failed with this.

./test: line 7: total: command not found

I don't know why.