If don't mind minor DIY wiring (no soldering required), here's one way to implement the external arbitration relay for less than $3 out-of-pocket (free shipping from Asia).
A single external arbitration relay can handle 2 (or more) ITADs and 2 (or more) crossing gates. Note that additional crossing gates do not need their BLUE wire connected to anything.
Off-the-shelf AC relays can be hard to find, and you might find you're paying more for shipping than the relay itself! Many ways to implement an AC relay using a more common and typically less expensive DC relay. You can add diodes or bridge-rectifiers. The nice thing about the eBay module approach is there are screw-terminals all-around so no soldering.
Here's the eBay module approach in greater detail:
The two modules are an AC-to-DC voltage regulator module to convert AC to 12V DC. And a 12V DC relay module. As shown, the relay module has 3 inputs on the left side; the lower 2 inputs are connected together. There is a jumper pin on the top which must be installed as shown to effect a so-called "Low" level trigger.
Note that this can also be used for insulated-rail triggering for accessories that are powered one way when "idle" and another way when "triggered". The crossing gate is a good example though I'd guess 2/3 aspect signals are probably the most common example.
