Can you limit a combo box's options based on the value of another combo box?

You can use this code as the custom Validation script of the second field (you didn't specify the field names, so I assumed the first one is called "Box1", adjust it in the second line of the code as necessary):

if (event.value!=event.target.defaultValue) {
	var box1 = this.getField("Box1");
	if (box1.valueAsString==box1.defaultValue) {
		app.alert("You must first select the first value.");
		event.rc = false;
	} else {
		var v1 = Number(box1.valueAsString);
		if (Number(event.value)<v1) {
			app.alert("You must select a value equal to, or higher than, the first value.");
			event.rc = false;
		}
	}
}