Is it possible to play two cards in the same field on the first step of a turn in Bohnanza ?
Yes,

You may plant both beans in the same field on any turn.

The rules state that during step one you:


Must plant the first bean

If it matches a currently planted bean then plant the new bean there
If you have an empty field plant it there
Otherwise you must harvest and sell all beans in one field before planting the new bean in the newly empty field

You may plant the second bean using the same rules as the first bean.


When planting the second bean, it is legal to harvest and sell the bean you just planted.  It's often a very good move to do so.