Boom! Figured it out...
 
SELECT tblCurrencies.CurrencyCode, 
    tmp1.SumPay, 
    tmp1.SumPayUSD, 
    tmp2.SumRec, 
    tmp2.SumRecUSD, 
    ((SumPayUSD + SumRecUSD)/(SELECT Sum(tblTransactions.USDAmount) FROM tblTransactions)) AS Proportion
 
FROM (tblCurrencies....
 
Completely forgot I...