Short Answer

To scan from your second scanner:

simple-scan hpaio:/net/officejet_pro_6970?ip=192.168.8.20

Long Answer

From Debian’s SaneOverNetwork:

##Scanning with a Network HP all-in-one (aio)-1

To scan over the network from a scanner on an HP aio (one which is not
connected by USB to a computer) you need only to install libsane-hpaio
(without its recommended packages) and pass the URI of the scanner to
the frontend. A non-free plugin might be required for the scanning
function.

The format of the URI is:

hpaio:/net/<aio_model_name>?ip=<IP_address_of_the_aio>  

This URI can be given directly to the frontend. It can also be
provided automatically to the frontend if either a print queue with
the hp:/… backend is set up or mDNS broadcasts for the scanner are
done by the aio. Automatic discovery will happen via the print queue
if both it and mDNS are available as discovery methods.

The IP address could be known from the way the aio’s networking was
set up; or it could be got from the aio’s front panel or deduced from
the output of

/usr/bin/lpinfo -v  

Each printer model supported by the installed verion of libsane-hpaio
is listed in /usr/share/hplip/data/models/models.dat. Model names are
enclosed in square brackets; like so, [envy_4500_series].

A frontend can be started with (for example):

simple-scan <URI>  

Typing the URI each time can be avoided by exploring what your DE
(Desktop Environment) or WM (Window Manager) offers for customising a
command. For typing from a terminal you might find a simple alias
sufficient.

There are more options for setup as the link describes. The information on alias to save typing might be very helpful.