From the LNB you've got two ports that are for the Exploras and four that are for normal decoders, take a cable from there and run it to the HD decoder and plug it into the LNB port on the decoder, take a cable from the rf-out of the explora2 to the rf-in on the HD decoder, take a cable from the rf out of the HD decoder and plug it into the TV (this is so you can watch the explora content on the TV) otherwise you'd use HDMI just for the connected decoder.
Set the Explora settings to RF-out and the HD to rf-in, you'll have to also fiddle with remote settings, think you set remote 2 to relay-respond.
Contact DSTV and tell them you'd like to activate Xtraview, supply the details they ask and bam