I might think that he may not sow one (variety) by itself and another by itself; it is, therefore, written "together", but each by itself is permitted.
(Variantly:) "Kilayim":To impose liability for both "vineyard" and "field."
Variantly: "lest tikdash the fullness of the seed": lest there be forbidden, as R' Yehuda has said.
"which you shall sow": This tells me only of what he himself sows. Whence do I derive what his neighbor sows (in the former's field) and which he would like to sustain? From "which you shall sow" — in any event.