In theory, yes you can. 2Do acts like an email client and so this is analogous to setting up your email account on more than one mac at the same time. The only downside is that one device may compete with another when trying to capture and sync this task across to the other device. If the iPad and iPhone both capture the same email, they may try processing it at the same time, leading to needless traffic. In an extremely rare case, this may even result in a duplicate on both devices (although there are checks to avoid this).
We recommend you set up one unique email account on a single device. Since you can set up two to three accounts on a single device, that should suffice. This way, you can set up two more accounts on your other device, effectively having 4 to 6 accounts on two devices. That’s a lot of emails!
NOTE: 2Do will NOT automatically sync and setup email accounts for you from one device to another. For your security, 2Do neither saves your credentials nor communicates them outside of your device. They’re completely secure, encrypted on disk, and in case of Gmail / Yahoo and Outlook, your credentials are verified by respective email providers and 2Do is only given temporary authorization to access your details without knowing your password. If you need to setup the same email account on another device, this will need to be carried out manually.