If you are transferring from the NRE a/c of your wife to your NRE a/c, there is no issue. When you are buying or investing that money, the income on such investment shall get included in her income, if such income is chargeable to tax in India. For instance, if you buy a house in India out of the funds transferred by your wife and let it out, the rental income on such property will be clubbed in her hands.
As the transfer of proposed transfer involves her money as well as your money, it is advisable to have two separate transfers to ease the workings for determination of tax liability, if any, on clubbing of incomes.