Retention agreement for a JV before the employment contract

Hi.
My main company employer A has created a new JV with another company B. I was selected by mgt to move from my company A to the new JV. No contract has been offered yet and they told me you will remain employee for A seconded to the JV. Yesterday, i received a retention agreement from the JV asking me to sign to get a bonus after a specific date if i stay with JV. My question: legally, can I sign retention agreement with the JV while I am still employee of the main company A ? I found this strange in fact. I was expecting to close the current contract with the company A, get new employment contract with the JV, sign it, then sign the retention agreenment after. Is that correct ? Thx for your help.