First of all, you should always use the latest version of ImgBurn. Older versions aren't supported.
16x is only reached near the end of the burn (i.e. when the burning laser is near the border of the disc). If there isn't enough data to completely fill the disc (e.g. less than ~4gb) then it won't reach 16x because it won't need to burn near the border.
You can add as many SATA burners as your system can handle and one won't impact performance of the other. As for opening several instances of ImgBurn, the bottleneck will be the HDD(s) where the image/files are being read from: if it can't keep up with the speed then there will be buffer underruns.
You can use the burners in sequence by using the Queue feature, have you looked at it?