That's exactly what I did two days ago. It seems to work most of the times. However, your preferred server seems to be listed as #2 in the pihole Settings (upstream Servers) (see here.
I then can see in the dashboard that arround 5 % of the queries use the "fallback" server specified as #1 Server.
However, I am not entirely sure if the "strict-order" options works like I would expect it do to do. I would expect it to try resolve the query first with the main upstream server. If this doesn't work, it automatically chooses the fallback upstream server. Nevertheless, sometimes the DNS seems to be not resolved at all and a manual refresh in the browser is required.