If you could get an unvented system that's short enough, you might put
two in parallel. You'd still have to have two lots of safety devices
(pressure relief, T&P relief valves, discharge pipes, etc., etc..) and
they should/must be serviced annually, so this could have high
maintenance costs. Probably a last resort solution.

I've seen unvented systems installed in an enclosure constructed at top
floor level, in the space over the stairwell. You have to have 6.5ft
ish headroom over the stairs at the lower levels, but the space in the
stairwell at top floor level is usually wasted. Probably impractical
for listed building dwellers.

You also need to minimize the distance for the discharge pipe from the
2 relief valves' ; it needs to be fairly direct. The pipe size
increases with it's length and the number of elbows. You can wind up
with a 54mm copper pipe on the outside of the building or it could be
impractical.