Current version of nut_webgui connects to a single NUT server, and monitor all UPS devices from that server.

However, if you have to use multiple NUT servers, you can use the repeater driver. For example:

Server A: /etc/nut/ups.conf

[apc_ups_A]
    # ...
    # ...
    # ...

# dummy-ups in repeater mode
# Ups show-ups in Server A, and all information is read from Server B via the network.
# For more details see: https://networkupstools.org/docs/man/dummy-ups.html#_repeater_mode_2
[apc_ups_B]
    driver = dummy-ups
    port = "apc_ups_name@server_b_address:3493"
    desc = "ups from other server"

When you connect to Server A with nut_webgui, you should be able to see UPS from Server B. This setup works fine for monitoring, but repeater driver has some limitations. Current driver implementation doesn't support instcmd and setvar operations (networkupstools/nut#94).