Successful 8086 projects were usually written in assembler - no way to get the speed and size down otherwise. I'm pretty sure Lotus 123 was all in assembler.
I'm not an assembly programmer and not very familiar with how that world works, but even then, if the two OSs were for the same architecture (x86), couldn't you still have a cross OS main part and then specific parts that deal with operating system things? I normally think of compiled languages like c being an abstraction over cpu architecture, not operating system api.
Yes, you can have common assembler code among platforms, provided they use the same CPU.
From what I've seen of code developed in the 80s, however, asm code was not written to be divided into general and os specific parts. Writing cross-platform code is a skill that gets learned over time, usually the hard way.
