What a pairing! I have been lucky to see Roberta Maxwell play Rosalind and Lady M. and the wife in Tartuffe at the Stratford Festival (Ontario) and she was wonderful! She is still wonderful today, as she showed in the Richard Nelson trilogy most recently. Perhaps it's good she never became a big star because she has been "our secret" as a stellar stage actress. (Despite her turn as Mrs. Oyl in Robert Altman's trippily-weird Popeye movie.)
I wonder how much Shaw she has done? I know she played Vivie Warren at the Shaw Festival (opposite Kate Reid).