Glenn Herbert wrote:A second bell would only be useful for extracting more heat into a mass, which is irrelevant to your case. You would want to have the exhaust from the bell below the bottom of the oven so that the hot gases flow all around it, but much more does not seem to be useful for your purposes.
The temperature at the top of the second chamber would be less than the temperature at the bottom of the first chamber, and several hundred degrees lower than the temperature at the top of the first chamber. The second chamber might be cool enough to run a copper pipe through for water heating, but only if the pipe is not pressurized and cannot be shut off at the outlet. If it can be shut off, you risk a steam explosion which could potentially kill someone.
You definitely want your chimney to extend at least several feet above the top of your riser, or you are likely to have problems with lighting and draft. Five feet above the riser would give me confidence in reliable functioning. Also, since the exhaust will be hot yet hopefully smokeless and invisible, I think you need to have the exhaust point inaccessible for safety.