I have often wondered how the radio manufacturer's build a twin band radio receiver, able to monitor both bands at the same moment.
Well I have a solution that on the face of things, could work for a Tri-band simultaneous monitoring.
The digital oscillator si5351A, has three internal vfo clocks, one of each could then be used for each radio receiver, 1st tuned to 40m, 2nd tuned to 20m and may be the 3rd t say 10m band.
How there would be the need for three I.F. Stages, as a second Arduino Uno or Nano, could the be used to control another si5351a for the BFO settings.
The combination of the main micro-control, the one connected to the local oscillator vfo digital oscillator, could use the I2C or the TWI system to talk to second the micro-controller setting the BFO for each of the monitor receivers.
The main Arduino Uno or Nano controller, could be the connected to a keyboard input say a 16 key keypad, “ 0 – 9”, and “A to D”, as well as “ * and # ”.
The keys “A – C” could be used to select the monitor receiver, that is one out of the three radio circuits. The “ * or # “ could be used to select if the radio selection input for the vfo or the bfo setting, with the “D” key as the enter button.
May perhaps use the “#” for the vfo setting, and the “*” key for the BFO selection, SSB to AM or FM etc, once the “ A – C “ radio selection is made, the “D” as the entry button.
Perhaps a 20 character 4 line display could be used, simple to use and with minimal coding.
To select the appropriate RF band pass filter, a TWI parallel output bus expansion chip could be used, two 8 way chips for the band selection, with three more 8bit parallel expansion chips, one for each radio to generate the DAC vari-cap tuning voltage for each of the front-end RF BPF.
Alternatively, have a bandpass filter to the width of each of the ham band, i.e, the 20m band filter would 350KHz wide, but able to switch feed all three radio receivers, or individual switch individual band tuned front BPF to any of the three radios. Relays could be used, or analogue switches or diode switching.
Alternatively, three sets of denco coil LC tuned type circuits, one set for each radio. I found a pdf reference for denco coils and circuits, be thought-full though, please do not over load this hams website. http://www.g4dmp.co.uk/dencocoil.pdf