I would like to score a climactic moment in a piece as follows.... a 40 person chorus, (assume a professional adult group) will be singing fortissimo clusters. Can I expect two notes struck simultaneously on standard orchestra tubular bells to be prominently heard?
Tubular bells can be very loud. They can cut through a full orchestra, so what you are proposing should work.
For an example of this kind of writing you might want to listen to a recording of Leonard Bernstein's "Missa brevis" (for chorus and percussion) which uses bells prominently.(particularly at 2:37)