The switch for channel 3 and 4 is the modulator output channel, set
one to channel 3 and the other to 4. You can use a mixer (a passive
splitter reversed will do) to connect them to the TV, now you can tune
each. If you are planning to get a third box you will have to look for
a modulator that outputs to VHF III or UHF.
Keep in mind that placing these devices to standby will usually turn
off their modulador leaving free channels for other modulators and
will reduce wasted power.