在两个下拉列表上交替约束的最佳方法

I have two dropdown selects that have the same values. I am building a converter from the source (left) unit to the destination (right) unit . It doesn't make sense to convert to and from like units. I know how to add an event on a dropdown select to remove the selected option from the alternate box but I am unclear on writing this to produce the best UI experience. Give the following trimmed case:I have two dropdown selects that have the same