The 896HD has an ADAT IN port which you can connect one of the ADAT outs of the 428 to. This way all 8 channels of the 428 converter will be transmitted to the 896HD. Note that the 896HD only supports 8 channel ADAT at either 44.1 or 48KHz so you will not be able to use 192KHz when connecting via ADAT.
Alternatively, you could route two of the 428 channels to the 896 via AES/EBU - the 896 will support up to 96KHz with an AES connection. You will need a 9-pin to 4 x XLR breakout cable for this (you can order one from any Focusrite supplier). If you want to record at 192 KHz with the 896 then you will have to connect via analogue from the 428 to the 896 analogue inputs since it is only the analogue inputs on the 896 that you can use to record at 192KHz.