Redirection of standard and error output appending to the same log file

I need to collect the standard output and error log from several processes into one single log file.

So every output must append to this log file.

I want to call all the jobs with lines like this:

$p=start-process myjob.bat -redirectstandardoutput $logfile -redirecterroroutput $logfile -wait

Where do I have to put the information to append?