Separate stoves gives you more control, and also allows you to fire up only what's needed (may use less fuel if you consistently do one operation more than the other).
A single, all-purpose unit that's always used in both capacities could theoretically take less fuel, since you would be reclaiming some waste heat from one process to heat the other.
One thing I've seen on masonry heaters is a cooktop where the firebox is below. After the fire goes out, you can use the firebox itself as an oven. Works great when space-saving is a priority. Drawback is that often a clean firebox is relatively small and well-insulated. To make a bigger oven, with more heat-retaining capacity, means the firebox may take longer to heat up or require more fuel to burn clean.
I was recently reading about someone building a stove-over-oven with a layer of brick to separate the stovetop, so that the stove could be turned down with a bypass flue damper.
Who was that.... here it is, Eric Shcroeder from Maine:
http://radiantstoves.com/blog/2012/12/combination-cooker-heating-stove-report-1/http://radiantstoves.com/blog/2012/12/combination-cooker-heating-stove-report-1/
Here's a small adobe cookstove, very basic, from India. If you swapped out the bricks on one end for a door, and blocked off one or both chimneys with some spare bricks near the end of the cooking fire, it would probably make a decent bake-oven for one or two loaves of bread at a time.
http://e-goodstove.blogspot.com/
For larger-scale feast preparations, I have been getting updates from Grant Steven in NZ and like what he's been playing with.
He cites this site as one of his inspirations for a large-scale cooktop:
http://www.appropedia.org/Modified_Justa_Stove
Here's Grant Steven's YouTube channel, as of now:
https://www.youtube.com/channel/UCpjI5aj7_6iPQ_QBuDM0vag
The
Approvecho folks and allies have done a lot of cookstove stuff, but they seem to favor lighter-weight or metal-based stoves. Ecept when building on-site in areas with more clay brick than scrap metal, once in a while.
I've seen a bunch of different variations on a light-weight pizza oven over a rocket firebox:
http://somewhatsketchy.com/?p=115,
http://greenrocketoven.com/,
http://www.koanga.org.nz/low-mass-insulated-barrel-oven/
For pizza they generally work pretty good. For bread or delicate baking that requires more even temperatures, artisan bakers often prefer the heavier-walled traditional wood-fired ovens. These are not efficient at all unless you are running them every day (it takes a lot of heat to warm that mass up from cold, but not a lot to re-heat it from warm to hot). But they produce ridiculously tasty artisan baked goods, and can take the large and lumpy firewood that might not fit in a small rocket firebox. (It could also be used to make hugel beds, so don't feel obliged to build one of these just to dispose of lumpy wood.)
Here's our massive double-chamber cob oven, burns a little cleaner than the varieties without a smoke chamber / chimney:http://www.scubbly.com/item/75812/instantlink/?affid=8105
You can also add a "smoke visor" over a simple dome oven as an afterthought, but unfortunately I can't find Grant Steven's demo just now (the link's broken).
Hope that helps.
-Erica W