Your best bet is to use
printf. You have two strings and you want to output them with some additional formatting. That's exactly what
$ printf "%s @@ %s\n" "$(cat /proc/loadavg)" "$(date)"
tr attempt does not work since
tr modifies characters, not words. You could use it to replace the newlines with one single character though:
$ ( cat /proc/loadavg; date ) | tr '\n' '@'
... but it doesn't do quite what you need.
sed attempt does not work since the newline is stripped from the input to
sed -n '/\n/p' inputfile would never print anything).
You could still do it with
sed if you read the second line (from
date) with the
N editing command while editing the first line (which will place a newline between them):
$ ( cat /proc/loadavg; date ) | sed 'N;s/\n/ @@ /'
... but I would personally prefer the