Yes you should be able to use the Dual() function for that purpose.
Dual(1/2*100 & ' %', 34) will display as 50 % while the number part will be 34 just to show that the text part can be entirely different from the numeric part of a dual value.
So in relation to to your expression it might have to look like this:
Dual( Sum(Amount) & ' ' & Num(Sum(Amount)/Sum(TOTAL Amount),'0%') , Sum(Amount) )
The only thing to be aware of is that the text part and the numeric part has to match up so that for every unique number there will be a unique text or it might not work as intended.