I don't think you can do it without extra components.
If length between the two ITADs is long enough (or the train is short enough) that neither ITAD is triggered when the train is between them, you need to add "memory" to the system.
If you use the time-delay of the ITAD, you can fiddle with the timing so that Red is held a bit longer - possibly long enough for the train to reach the other ITAD to then hold the Red. But for the general case where the train can stop between ITADs, using time delay as a proxy for memory would not work. This "OR" logic (either or both ITAD triggers the MTH 3-aspect signal) could be implemented with, say, $5 of parts such as relay, diodes, capacitors.
But for the general case where the train can stop or go really slowly between ITADs, implementing a memory circuit is more complex and I suggest you google "model train bidirectional signal" for companies that make off-the-shelf modules for bidirectional crossing signals and the like. Most of these suppliers sell their own sensors so some homework is required to see if they work with the MTH ITADs. You could cobble together a DIY memory circuit with digital logic chips and/or latching relays. The components aren't that expensive compared to the cost of ITADs and signal bridges, but the wiring/soldering could get tedious.
