This should do the trick:
'the last day of the next month:
[DueDate_30dEOM] : dateserial(year(dateadd("m",2,[TaxDate])),month(dateadd("m",2,[TaxDate])),1)-1
'add 2 months, find the 1st day of that month, and subtract 1 day
'the last day of the month after that
[DueDate_60dEOM] ...