What do you mean by enforcing an utxo? Maybe I misinterpret your question.

One utxo to open a channel and another one to close it. What happens in between on LN has nothing to do with mainnet, just like opendime. We can exchange as many opendimes as many times we like until we spend it doesn't require any utxos. Probably not the best example, the usual one is the concept of bar tabs.