How to cheaply switch between two sets of 7-input-wires?
Truly horrifying circuit, by the way. The transistors are drawn backwards (he managed to get them right on the one he built for himself). 

It will also cause electrolysis of the probes if you leave it on all the time since he's using DC to detect conductivity. But it is simple. 

Frankly, I think it would be easier to build two of these than it would be to switch the probe lines. Or you could wire the probes to something that fits a switch box. 

If you insist, it can be done with three 74HC4053 triple analog multiplexers (that will handle up to 9 lines). Increase the resistors to 1K from 33R and put one on each probe input to give the chips a chance of surviving when bad things happen at the probes, and put the chips in sockets so you can replace them when they inevitably get fried. 



Ground E (/Enable) and \$V_{EE}\$ (and GND, of course), \$V_{CC}\$ is your 5V supply, connect all the \$S_i\$ together with a 10K pullup to +5, and a switch to ground to control the analog switches simultaneously. Common from each switch goes to a transistor base, and the other two inputs go through resistors to the probes. 

If you run this crap outside, not only could you kill the muxes at the first electrical storm but you might invite Mr. Lightning in for a visit.