When should one use дві instead of два? Is it only directly before a feminine noun? Or also when a feminine noun is implied? That is, if the person here is asking for 2 of something fem, then would it be Принесіть дві, будь ласка?
The later is true. If the person were asking about something feminine, then it would be "дві." "Два" implies something masculine/neuter.
Дві, два and двох are accepted now, which are all the possible Accusative forms.