You will have 3 fields. All can show at all times.

1- [Enter Amount]

2- [Weight UOM] - Multiople choice of either "KG" or "LBS"

3- [Alternative Amount] - Formula text that will evaluate what UOM you selected and show the opposite.

Of course you will have to have the math work both ways. A case formula should do it;

Case([Weight UOM],

"kg", (math to convert kg to lbs) &" lbs",

"lbs", (math to convert lbs to kg) &" kg",

"")