yes, had this issue several times. As I understand a voucher starting 074 can only be used on KLM Website, while a voucher starting 057 can only be used on AF Website. As the prices are (usually) the same, it did not make a difference for me so far.
the issue is when you want to use two vouchers - one issued by KL and one issued by AF - you can't do that online, but need to call. best option is to proceed to the payment page. by then you got a PNR - you can use the PNR when talking to the phone agent without the need to assemble the journey again via phone.