Yes, you can do multiple tanks with a single appropriately sized Tank Sealer Kit.
To do this, be sure to prep both tanks well and then pour part of the can of Tank Sealer into one tank and then the other part of the can of Tank Sealer into the other tank immediately. Never drain the Tank Sealer from one tank to another.
KBS Klean and RustBlast can be used on multiple tanks, and if needed, can be poured from one tank to the next. You can typically prep a few tanks with KBS Klean and RustBlast without losing any effectiveness.
Please read the complete Tank Sealer Application Directions before you begin your project.