This may be an overcomplicated solution but maybe an if statement in the second field you want to populate.
In the field you want to populate quantity place a script similar to this. Bear in mind this will require a few lines of script per value you want bound.
Dropdown.rawValue == 1
$.rawValue == Quanity1
dropdown.rawValue == 2
$.rawValue == Quanity2
this code basically says if dropdown value is 1, this field will be "1" where "1" is whatever value you want associated with dropdown value 1. Repeat this same step infinitely for as many values associated with the dropdown including repeated values. ie. if dropdown is 1 then field value is 1 and if dropdown value is 2 field value is also 1.
I hope this will help get you on the right track. I am no expert( rather far from it as I have only been using livecycle for about a month) but I have managed to learn a lot and hope to share that with other trying to learn it.