Hi Patrick,
It does look like you've got a reasonable location in mind for the rocket mass heater - near the center of the space to be heated, and with the bench forming a useful piece of built-in furniture.
A few things I'd like more information about:
-- The foundations / load-bearing for the mass heater.
It sounds like there are a pair of existing, masonry, load-bearing walls below the lower hearth, and the upper hearth is also partially supported by the masonry wall of the garage below?
Although adding the rocket mass heater in place of the existing bench may change the weight location somewhat, it shouldn't be a huge change in the total weight to be supported. You might need to expand or beef up the support directly below the combustion area of the new heater.
-- Clearances to combustibles: It will simplify several things if you can remove any combustible materials in the immediate area around the rocket mass heater, for example by replacing the half-wall there with a masonry half-wall in the same style as the fireplace. It might pay to examine the floor structure from below, and see if it's practical to cut out a 4' by 5' box and re-frame the floor joists so you can support the heater directly on the masonry (non-combustible) structures below. People have built rocket mass heaters over wood floors before, it's not impossible, but it does turn out to involve a bit more work to protect the floor from the heat.
I don't think you'll gain much by removing the lower fireplace insert. To clear enough room for the new rocket mass heater could involve removing structural elements of the existing masonry.
I realize you're already committed to a fair amount of masonry alterations to hollow out the existing bench. I congratulate your ambition and hope you find the existing bench happens to be hollow; it would save a lot of work.
I suspect the structure of the two chimneys is tied in rather more closely than the bench. You'd risk possibly cracking part of the chimney lining if you do the work with rough-and-ready tools. Do you have access to a masonry saw or drill for cleanly cutting into the existing masonry? They can often be rented, or a local mason might be willing to do this aspect of the work on your behalf.
Making the rocket mass heater on two levels is well within the design parameters of rocket mass heaters, and has been done before.
-- The other thing I'd like more information about is the location and climate of the house. If you can find your nearest (climate-similar) weather station from this database:
http://www.huduser.org/portal/resources/UtilityModel/hdd.html
and describe briefly the insulation and wall structure of the house, it will help me give you a better idea what size heater may be appropriate. I understand the whole building, both levels to be heated, is about 600 SF with an unheated garage/basement below?
-- Finally, do you have any concerns about maintaining the building's insurance or up-to-code status? Rocket mass heaters are kind of an oddball category, some insurers respond well to them while others can be reluctant to insure them. Same with local building code offices. The heaters are exempt from EPA by weight, and most (but not quite all) of the building methods can be done so as to comply with the Masonry Heater ASTM standard. (There are a couple of points in the ASTM standard that are ambiguous when applied to rocket mass heaters.)
If you would need local approval in order to pursue the project, we can offer some documents by e-mail that might help with the initial conversations with any officials / agents.
Yours,
Erica and Ernie W