There is no reason to suppose that code written in one assembler will "just work" in another.

You really have to study the manuals for both assemblers to understand the differences and, thus, know whether code written in one will be portable to the other.

The same even applies to 'C' - especially at the kind of level typical of 8051 projects, where you are generally highly dependent on implementation-defined language features, and extensions to the standard language.

There are well-established techniques to help you make your code easily portable - but it certainly does not "just happen".