It certainly is possible. The cardinal rule of Bohnanza planting is "you must plant your first card, you may plant your second card". The only restriction on those two plantings is that they must both be legal. And you can harvest at any time, even for no profit, even between one planting in the first step and the next.
Initially I misread your question and answered as if you were asking "is it possible to play two cards of the same type in the same field in the first step of the first turn", which of course it is. Simply because it would be totally bizarre to overplant in the same field.
(Someone will doubtless correct me if I'm wrong, but I think there's no possible advantage to keeping the second bean field free in this way in the first turn. If you plant and then overplant, and then turn up two copies of the bean that is now in the discard pile, or someone (mystifyingly) wants to give you three free copies of the bean in the discard pile, then you have presumably lost out! The absolute worst case scenario is that you will have to overplant the first bean field later in your first turn, or in your second turn, but that's not actually a very bad scenario at all: you lost nothing by letting the bean hang around in your field for a short time, while you were considering your options.)
Later on, it's possible to rapidly plant and overplant to cycle through your beans. Just remember that you can never plant over a single bean if you have another field planted with multiple beans!