The problem is with your assumptions of multi-port code using different assemblers. Old 16/32 bit MASM is not compatible with 64 bit MASM which is a separate stand alone tool. I have no problems at all running the current version of 64 bit MASM but then I am not trying to run legacy incompatible code with it, I only assembler Microsoft 64 bit MASM code with 64 bit MASM. MASM in 64 bit does not support either 16 bit or 32 bit MASM code, for that you use 16 or 32 bit ML.EXE which has been around since 1990.
Since VC/VS is the major development environment at a world level, its version of 64 bit MASM is the reference for MASM compatibility, for the little that its worth, 16 bit real mode MASM code does not build with the 64 bit version of MASM either. The Watcom clones under development are not MASM 64 compatible which is of course their right to develop whatever they like.
Since I can tolerate many different tools, FASM is not ML 32 compatible, neither is GAS, Jeremy Gordon's assembler, Bogdan's assembler, there is a 64 bit Russian version of TASM that is 64 bit from memory, all which are not 32 bit MASM compatible.