Grab the list from the servers

rpm -qa --queryformat='%{NAME}-%{VERSION}\n' |sort > server1.txt
rpm -qa --queryformat='%{NAME}-%{VERSION}\n' |sort > server2.txt

Copy the files over to where you want to compare, probably one of the servers in question.

for R in $(cat server1.txt) ; do grep $R server2.txt >/dev/null || echo $R ; done