With the introduction of digital TV signals in recent years TV and radion channels are now delivered as groups of channels, or multiplexes. There are currently 8 different multiplexes available in the UK which are delivered using a range of frequencies to achieve the necessary geographical coverage.
You will need to check with your TV aerial installation company that all of the multiplexes are available in your local area, or you can use this website.
Each card in a DVB gateway has four independent tuners which can be tuned any of the available multiplexes. Once a multiplex has been selected and tuned then any / all of the channels on the multiplex can be delivered by the IDT system.
To decide whether you need a single or dual card system all you have to do is decide how many different multiplexes you need channels from.
Just like DVB T/T2 digital TV broadcasts, satellite channels are also broadcast as groups of channels or multiplexes. However because of the much wider geographical coverage of a satellite broadcast the multiplexes tend to carry all of the regional variations of a single channel. Your satellite installer will line you dish up to point at the appropriate satellite, typically Astra for Freesat in the UK.
Using a quad LNB and a multi-switch each DVB S/S2 card in the DVB gateway can be tuned to 4 multiplexes. This website is a really useful tool for establishing what multiplex a channel can be found on and establishing the necessary tuning information required by the DVB gateway. You can also use this website that lets you look up a transmitter and check the reception.
DVB Gateway cannot deliver encrypted channels and if these are required by the system then a satellite decoder will be required and the HDMI output of this can be encoded using an ENC002.