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You do not need paste (or quotes for that matter) at all:
expression( A[2]~and~B[2] )
Test:
plot(1,1, main=expression( A[2]~and~B[2] ) )
The syntactic principle is that tildes (which creates a space) and asterisks (non-space plotmath separator) are used to separate items and that no quotes are needed unless you are using a plotmath function name .... such as wanting the word "paste" or "sqrt" to appear in the displayed version of the expression.