You can actually hold more than one lightning lane at a time…you can select a new one every 2 hours, whether or not you have used the previous one. Depending on your return time, you might not have used one that was for more than 2 hours away when you booked it.
Additionally, you don’t have to wait 2 hours to book your next one if you use it before the two hour mark, you can book another one right after you use it to enter the lightning lane (FastPass line).