[7] "This principle is not contrary to the general rule that 'parol evidence is admissible to establish the absence of an intention to make a gift of either separate or community property, although the instrument of conveyance is made by the husband to the wife alone or as joint tenant with him or is from a third person to the wife or to both as joint tenants at the husband's direction.' (Huber v. Huber, 27 Cal. 2d 784, 788 [167 P.2d 708].) Such evidence always is admissible when it tends to prove a mutual understanding or agreement that the other spouse was not to receive a present gift under a conveyance of property to both spouses as joint tenants. (Huber v. Huber, supra, 27 Cal. 2d 784, 789.)"