No, credits are not shared between the two products. Email credits are specific to each one: Sarbacane Campaigns credits cannot be used in Sarbacane Sendkit, and vice versa. SMS credits are only for Sarbacane Campaigns, since Sarbacane Sendkit does not send SMS messages.
The Usage section on your account settings home page also displays a separate balance for EMAIL VIA CAMPAIGNS, EMAIL VIA SENDKIT and SMS.
