Nested tabularx and itemize => TeX capacity exceeded [input stack size=5000]

• The column specification

  >{hsize=dimexpr2hsize+2tabcolsep+arrayrulewidthrelax} X
  

  is fundamentally flawed. (The +arrayrulewidth part is wrong from a purely computational point of view (since your tables don't feature vertical bars), but it doesn't cause the error exception.) Replace the column specification with

  p{dimexprtextwidth-2tabcolseprelax}
  

• Why the claim of fundamental misspecification? The tabularx package allows users to specify unequal columns widths for columns of type X. However, several sanity checks have to be satisfied. First and foremost, the sum of the relative column widths has to sum to the number of columns of type X. E.g., for a tabularx enviroment with 2 columns of type X, >{hsize=1.333hsize}X >{hsize=0.667hsize}X is fine since 1.333+0.667=2. Observe that your code does not achieve 2hsize; instead, it achieves 2hsize+2tabcolsep+arrayrulewidth. This discrepancy alone should tip you off to the fact that something is wrong. The fundamental mistake you're making, then, is that you're trying to use a single column of type X in the second argument of multicolumn. Hence my suggestion to use a single column of type p.

• Next, the overall width of the "inner" tabularx environment is wrong. Replace

  begin{tabularx}{textwidth}{ X  X }
  

  with

  begin{tabularx}{hsize}{XX}
  

  Why? By construction, textwidth exceeds hsize by 2tabcolsep.

• Last but not least, don't forget to insert noindent immediately before the first, or "outer", tabularx environment -- unless you want to be be bombarded with warning messages that the tabularx environment is too wide.

The following code compiles. That said, I'm certain that it'll ever win a prize for code beauty. For the life of me, I cannot fathom the purpose of the nested tabularx environments. I would never code anything like that in my own LaTeX working papers.

documentclass{article}
usepackage{tabularx} 

begin{document}
noindent % <-- very important
begin{tabularx}{textwidth}{XX}

multicolumn{2}{ p{dimexprtextwidth-2tabcolseprelax} }{
   begin{tabularx}{hsize}{XX}
      A & B 
   end{tabularx}
} 

begin{itemize}
  item One item
end{itemize}
 &  ABD 
end{tabularx}

end{document}