Do you mean the subreport only displays in detail_b? Then use "format multiple columns" within the subreport to achieve this.
Otherwise, if you were trying to do this with fields, I think you could use the "next" function and lay out the fields like:
{table.field} {@nextfield}//where {@nextfield} = next({table.field})
And then suppress every other detail_b section using:
remainder(recordnumber,2) = 0
-LB