There's some discussion of tenancy by the entirety for jointly held accounts in the case, Beal Bank, SSB v. Almand and Associates, 780 So. 2d 45 (Fla. 2001). The case is discussed more recently in a federal trial court decision:
Quote Quoting Branch Banking & Trust Co. v. Maxwell
In Florida, a married couple may own property as TBE. See Beal Bank, SSB v. Almand & Assocs., 780 So.2d 43, 52-53 (Fla. 2001). The six necessary unities of TBE property consist of: (1) unity of possession (joint ownership and control); (2) unity of interest (the interests in the property must be identical); (3) unity of title (the interests must have originated in the same instrument); (4) unity of time (the interests must have commenced simultaneously); (5) survivorship; and (6) unity of marriage (the parties must be married at the time the property became titled in their joint names). Id. at 52 (citations and footnote omitted). When real property is titled in the name of both spouses and the other six unities are established, an intent to hold the property as TBE is presumed. Id. at 54-55. A rebuttable presumption of TBE extends to personal property when the same conditions are satisfied. See id. at 57-58 (holding that presumption arises that a bank account titled in the names of both spouses is held as TBE if established in accordance with the six unities).
Under the federal court's interpretation of Florida law, the fact that an account was held before marriage does not of itself prevent the account from being TBE property if all six unities exist, and also the deposit of funds into such an account does not necessarily destroy the TBE status of funds that pass through the account. The earlier state court decision indicates that some banks contractually prevent their accounts from being held as a tenancy by the entirety, so if this is the type of argument that your ex- is arguing then your lawyer should seek a copy of the contract for the account. Your lawyer is also the best person to evaluate under the facts whether there would be a claim that the funds or account are plausibly held as TBE.

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