I'm running out of space in the Inventory. I heard some people saying about two email on One phone is not violate the TOS. But in the internet (most of reddit) says it's against TOS. I just wonder is there a way to have two Ingress accounts (One as Inventory) in one phone without violating TOS?
If you've got two separate email addresses for two separate people then that is not against TOS. One of them can log in on a device, log out and then another person can log in on another device. That is fine.
What is totally against TOS is one person having multiple email addresses for the purpose of having multiple Ingress accounts. This is called multi-accounting and it is a reportable and bannable offence.
What you need to do is clean up your inventory, recycle lower level bursters and resonators, use the XM to recharge portals, clear out your stock of keys, go on a smashing rampage and use up bursters. Buy key-lockers if you absolutely can't choose between your precious keys. 2000 items is more than enough space for a player.