Just for clarity, two unidirectional channels aren’t exactly equal to a bidirectional. But for XRP it isn’t much of a problem due to the speed of top up and channel close.
Bidirectionals are nice if you’re doing a lot of back and forth trading as they can continue indefinitely (within the collateral bounds). Two unidirectionals will eventually run out of spendable collateral.
Although as @JoelKatz mentions bidirectionals can’t do (non cooperative) instant closes.