One possible way for use :

1. create view
v_currency
Currency -(+)-> Null_Curr_Date

2. write select with union

mwebwork --> v_Currency
and
mwebExpense --> v_Currency
union
mwebExpense -(+)-> v_Currency
(where not exists mwebwork --> v_Currency)
union
mwebwork -(+)-> v_Currency
(where not exists mwebExpense --> v_Currency)

may this information will be helpfull.