Two comments. The prior failure was probably due to using x86 in automation to roll an x64 install, which will NOT ever work.
And the answer to your question is No. There is no intelligence that will tell PXE which architecture to load based on the next step in the job or task. There is no communication between the job and PXE whatsoever. You have to ensure that your x64 SOI jobs use x64 automation, and vise-versa.
Nice idea, and who knows, maybe someday, but today - no. And no, not in the 7.5 release either.