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",
"")