I split one liquid smack pack between two 3.5 gallon batches of 1.056 OG cider today. I know I under pitched. Is it better to pitch more tomorrow (18 hours later) or wait until it finishes and check the gravity and add more if under attenuated?
Pitch more now
I would do it sooner than later. You don’t want that yeast to be strained. Have you considered doing starters? It’s so easy and it ensures a healthy ferment
If those are the options I wouldn’t worry about it. After 18 hours you’ll be mostly if not entirely through the growth phase.
Did fermentation get going in a reasonable amount of time? If so, then no worries.