If you really can't link the fields then try this:
Create a variable vSelectionsA
'(' & Concat(DISTINCT Chr(34) & A & Chr(34),'|') & ')'
Then, in the OnSelect trigger for A, add an action for Select in Field, Field = B, Selection = $(vSelectionsA) without the equals sign.
I may have the syntax slightly wrong and can't test right now but this should get you there.
Hope this helps,