Yes, you can link up to three external bank accounts to fund your joint account. Additionally, you can always use Checking to fund your joint account.
Checking accounts and the Betterment Visa Debit Card provided by and issued by nbkc bank, Member FDIC. Funds deposited into Checking are FDIC-insured up to $250k for individual accounts and up to $250k per depositor for joint accounts. Betterment Checking made available through Betterment Financial LLC. Neither Betterment Financial LLC, nor any of their affiliates, is a bank. Betterment Financial LLC reimburses ATM fees and the Visa® 1% foreign transaction fee worldwide, everywhere Visa is accepted.