To use 2 LNB's on the same dish you will need at least a 60cm dish for say both the old and new Astra satelites if you live in the south. It would be better to use an 80cm if possible. The minidish is just not large enough.
There is no difference in LNB's for digital and analog.
If you say what satelites you wish to receive I could give more advice.
If you just want to recieve sky from two boxes then you can use the minidish but need a twin output LNB (around £50) to replace the single output LNB and a separate cable for the second output to go to your other SKY box.
