Easy ways:
1. Connect one STB with AV and another with RF
2. Use 2 RCA cables and connect one STB to AV1 IN and other to AV2 IN if TV has 2 AV IN ports.
If you want to experiment: (all these steps will work on FTA STBs not sure about RDTV)
1. Take RF out from both STB, connect to a signal modulator/ RF joiner. Take the output and connect it to TV
2. Take RF out from one STB, connect to RF IN of other STB (if it has one) and take the AV out of he second STB and connect it to TV
3. If the RDTV box is DiSEqC compatible and can scan FTA then use a DiSEqC switch and connect both LNB IN to RDTV STB and connect to TV.