If you're going to do this, you need to deal with the possibility that the script will die unceremoniously, leaving the lock file behind.  This adds a trap, makes the lock file identify the machine, PID, and date/time the lock was asserted, and examines that information and attempts to determine whether the lock is really valid rather than blindly assuming that it is.

#!/bin/bash
Lock=/some-where/rsync-is-running
Log=/some-where/rsync.log
if [ -f $Lock ]
 then  
  read <$Lock Machine PID RDate
  if [ `uname -n` = "$Machine" ]
   then
    if [ "x`ps -ocmd= -p $PID`" = "x$0" ]
     then
      echo "`date` - An instance of rsync is already running on this server:  `ps -fp $PID`" >>$Log
      exit
     else
      echo "`date` Ignoring invalid lock entry: `cat $Lock`"
    fi #[ "x`ps -ocmd= -p $PID`" = "x$0" ]
   else
       echo "`date` - An instance of rsync appears to be already running on $Machine: `cat $Lock`" >>$Log
      exit
   fi #[ `uname -n` = "$Machine" ]
fi #[ -f $Lock ]
# If we make it this far, actually do it.
echo "`date` - Starting synchronization" >> $Log
trap 'rm $Lock; exit' 1 2 3 4 5 6 7 8 10 11 12 13 14 15
echo `uname -n` $$ `date` >$Lock
# ... your script here ...
echo "`date` - Finished synchronization" >> $Log
rm $Lock

You could probably even use rcmd to validate the alleged PID of the process on the other machine, if you wanted to go to the trouble.  Or just have one server be the designated initiator of the rsync, which simplifies things greatly.