Most 12 yr old furnaces were 80% efficient and will remain 80% if
maintained. I would just turn off one tank and see if that does it ok
for you Chicago water is apx 45f now , turn off the second tank and
leave the first one in line to temper the water to room temp. I pulled
out a 20 yr old tank in Chgo recently it had 12" of sediment, and a
yellow flame it probably was down 40% in efficiency. Tankless are
expensive but save the most. I would look at R 50-60 for your attic
where you are. Be sure you get a written load calculation before you buy
a furnace. Alot of tank loss is up the chimney, if one will do it it
will be cheaper to run, I left in my tank as a tempering tank when I put
in mt tankless, my savings give an apx 4 yr payback.