You can use a standard multiswitch. The most common one that you'll find is a 3X4 multiswitch, where you have a Horizontal input, a Vertical input, and an input for your tv antenna.. then you have 4 outputs to go to your receiver.
I believe a SW21 works more like a 22k switch in that it uses something other than horizontal/vertical switching to switch between inputs. Anything other than a multiswitch that is designed to combine horizontal and vertical signals will require you to setup multiple satellites to access the different inputs. i.e. you could use a 22k switch or a diseqc (or a sw21 if your fta receiver could control one, which I'm not sure the actual control mechanism for a sw21 is supported by most/and fta receivers), but the way that FTa receivers are designed, you'd have to define a satellite for the first port of the switch, and another satellite for the 2nd port (most/all fta receivers don't support having multiple lnb's on the same satellite, which is what using one of those switches would basically indicate)
I have actually used a Primestar dish and LNB with dual ports (H,V) with a dish network legacy SW21 to combine both ports into 1 input and it works great. The only problem is you can't use that SW21 in conjunction with any other Diseqc switches - or at least I haven't had any success with it. I guess you could also try just a standard 22khz switch and that would work.