This is a specific case of the general problem of "radiative transfer," well studied in physics, engineering, and (in my area of interest) computer graphics. The key thing describing the process in my field is called "The Rendering Equation", and expresses the light arriving at a point of surface $B$ in terms of an integral of light leaving surface $A$, taken over all of $A$. There are multiple factors involved:
The distribution of emitted light from each point of $A$ (your "isotropically" appears to address this, but it's subtle; I suspect you mean that the emitted radiance from $a$ in direction $v$ is a constant function of $v$ (and possibly of $a$ as well).
The occlusion of points of $A$: not every point of $A$ is visible from every point of $B$, so there's a "visibility term" in the equation (one that typically cannot be expressed analytically)
A change-of-variable from integration over the sphere of arriving-directions-at-$b \in B$ to "integration over the surface $A$", which involves a couple of cosines of incident and exitant angles, and an inverse-squared-distance factor.
Self-reflection of $A$, and multiple-bounce reflections for $B$: light emitted from one point of $A$ may hit another, and then be reflected towards a point of $B$. Light hitting $B$ from $A$ may be reflected back towards $A$, and then back yet again towards $B$.
This last fact makes the rendering equation into an integral equation, in which the radiance field $L$ is related to an integral of $L$, just as in a differential equation like $y' = -3y + 2$, the function $y$ is related to its derivative.
Solving integral equations exactly is seldom possible; much of the field of computer graphics consists of approximating solutions to this particular equation in ways where the errors are (somewhat) bounded, and the time involved in computing the solution is (somewhat) bounded as well.
Short answer: this is just plain tough, and to do it well requires reading a lot. If you want an approximate but quick solution, just read any graphics paper from the 1970s. (But don't expect them to mention units like Watts per square meter steradian!)