Easily done with Enterprise forms. It's made to do this with its conditional visibilty features. Also, you can have the very same DropDownList bind to a different set of values based on the value of another element, if you wish.
For example, I built a table which holds building codes, building names, and building type. A user uses a radio button to select building type. Once a selection is made, there is an AJAX postback and another element is populated from a stored procedure that took a token which was replaced with the radio button value.
I can make the second element be invisible until there is a building type selected, too. So there is conditional visibility, and conditional databinding.
This is from a real-life form. It's on the second page of the form, which is not visible until folks make a selection on page 1 to ensure that they really need to fill out this form (It's a work request form). If they want a telephone, or Cable TV or washer or dryer repaired, and select that, they get a conditionally visible html element which includes a link to the appropriate place for such a request. Else, page 2 becomes visible, and they proceed.
So you have the option to have your values listed in different dropdown lists, and make one or another visible based on a selection in another element - or use the same dropdownlist and have it populated conditionally based on the value of a previous selection ... or a combination of both at the same time.
Enterprise forms is extremely advanced and can meet a significantly large percentage of the needs of a portal that has fairly complex business requirements.
It does simple stuff, too ... The first question I ask when I meet a new need or requirement is "can I do it in Enterprise forms?" The answer is usually yes ... not always, but usually.
Let me know if you'd like to send some data values my way and I'll build an example for you.