This is one of the shortcomings of the new high-resolution digital formats, the signals available are only analog.
There are a few options.
1) Take 6 Y adapters and connect them to the 5.1 bypass input.
2) Buy 3 stereo switch boxes.
3) Get something like the Sony TA-E9000ES (I think that's the part. It's a 3input, 6 channel analog preamp. You'd take the analog outs of the DVD-A and SACD into this, and the analog outputs of the preamp into the 5.1 input of your receiver. This piece is about US$700 at list, I don't know what it's streeting for.
4) The best, and most expensive solution, is the EMM Labs Switchman-2, which has four sets of 6 channel inputs, and is nothing short of fabulous. It's US$3200 and you'd be best served upgrading everything else in your system.
Ok, 4 probably isn't realistic, and with option 1, you have to be careful not to play them both back at the same time.
Ask me about bass management some other time ;-)
Regards,