TextIt does not support switching between SIM cards on a dual SIM phones, as Android does not officially support dual SIM phones. The Nyaruka TextIt Android application works with one phone number at a time, meaning that you're only able to use one SIM card per phone. If you want to use multiple SIM cards, you'll need to acquire multiple phones and install the Nyaruka TextIt Android application on each.
You may connect multiple Android channels to your account. Some of our users connect an Android channel for each carrier in a country, though enabling bulk sending through Nexmo will allow you to send messages at the same rate across all carriers.