It's not very clear but using two different devices would/could be considered cheating.
Read the "Cheating" clause on this page where it says:
Remember Ingress is meant to be played on a mobile device
Again one of this post's answers mentions a Niantic founder's (John Hanke) explanation on the matter but it's completely hearsay:
This matches up with what John Hanke described to some players at the recent Amsterdam anomaly (cut 'n pasted from one of those players)
iitc = no ban, no problem
mirror download = no
Google Play Store signature = ban
modded apk = ban
multi device = ok
multi account (via same Play Store signature) = ban
if you use another email on ingress, than you used on Playstore = ok
merchandising = OK as long as its reasonable.
if not reasonable, they ask politely to stop. and there could be an opening for licence
And this old post from Niantic's Google Groups era (even if it's not what you mean) warns about account ban.
Question: Has anyone tried log on one account on two phones at two different places?
What will happen now?
What should happen(official)?
Answer: Players have been reported/banned for this
I would suggest you not to use a second device and to buy a battery pack. It should be much easier to carry a battery pack than using a second phone.