upstream {
    server 1.1.1.1:8080;
    server 2.2.2.2:8080 backup;
}
server {
    ...

$ sed '/1.1.1.1/s/1.1.1.1[^;]*/& backup/' file

$ sed '/2.2.2.2/s/(2.2.2.2[^;]*) backup/1/' file

$ sed '/1.1.1.1/s/1.1.1.1[^;]*/& backup/;/2.2.2.2/s/(2.2.2.2[^;]*) backup/1/' file