There is no concept of joint account in Sukanya Samriddhi Account. Means one account is for the benefit of one child only. If you open the account for 2 girls then how the benefit will be shared?
You can obviously open Sukanya Yojana Account for your 2 girl child in SBI. But you have to open 2 different accounts in the name of them each. You can deposit money for both of them. Or you can open one account in your name as depositor and another in your spouse’s name so that you both can enjoy tax exemption under section 80C up to maximum limit, in case you both are working.