BillG claims Paul did most of the emulator work and he worked on the interpreter core. With the floating point routines by Monte Davidoff who they hired. It was a team effort of course, but BillG was adamant to take credit for the interpreter specifically.
From the source code itself:
PAUL ALLEN WROTE THE NON-RUNTIME STUFF.
BILL GATES WROTE THE RUNTIME STUFF.
MONTE DAVIDOFF WROTE THE MATH PACKAGE.