I was afraid of this. Would a call forwarding number from Call Rail be OK for the second location or is it better to stick with something like a cell # just in case the forwarding number is lost?I do indeed, especially if they're in the same location (even if it's hidden) and in a similar business category. Depending on the circumstances, it might fly without getting either one flagged as spam or a duplicate at least, but it will still likely have some negative ranking impact.
If you're sure you're not risking one of them getting flagged (you'll need to share more details if you want input on that too), then it just depends how much the NAP is out there for the two businesses. If one's new and still easy to change I'd go for it. If both are old and entrenched with a million mentions of the phone number, and you're not sure if you can clean up the NAP well enough, you'd be better off sticking with what you've got. There's plenty of businesses that use the same phone number and don't get flagged, but you never know what'll happen if a human moderator ever takes a look.