HAproxy request redirecting to multiple backends

I don't know, but try this in the backend configuration (adapt 10 with your maxconn) :

acl too_many be_sess_rate gt 10
use_backend b_too_many if too_many be_sess_rate

What you want to do is have an acl in the frontend send the request to a different backend based on the URL.

frontend shire
    bind *:80
    mode http
    acl path-frodo path_beg /frodo
    use backend longbottom if path-frodo
    default_backend bagend

backend longbottom
    mode http
    balance roundrobin
    option httpchk GET / HTTP/1.1\r\nHost:\ www.example.com
    server  app1_00 10.0.0.1:9100 check
    server  app1_01 10.0.0.1:9101 check
    server  app1_02 10.0.0.1:9102 check
    server  app1_03 10.0.0.1:9103 check

backend bagend
    mode http
    balance roundrobin
    option httpchk GET / HTTP/1.1\r\nHost:\ www.example.com
    server  app1_00 10.0.0.1:9100 check
    server  app1_01 10.0.0.1:9101 check
    server  app1_02 10.0.0.1:9102 check
    server  app1_03 10.0.0.1:9103 check
    server  app2_00 10.0.0.2:9100 check
    server  app2_01 10.0.0.2:9101 check
    server  app2_02 10.0.0.2:9102 check
    server  app2_03 10.0.0.2:9103 check

All request beginning with /frodo will hit only app1_00, app1_01, app1_02 and app1_03. All other requests will hit everything.