Perhaps an easy way to sort it out is whoever gets tapped onto first gets the fare. Yes, in the AM peak the TTC would be losing out on money, but in the PM peak (assuming it's a reverse trip), all of those people would be tapping onto the TTC first. If the "he who gets tapped onto first gets paid" policy is used, each company would get the fare revenue from one of those two trips. For the TTC, that may actually end up being a net plus, because a lot of those people today choose to walk because they don't want to pay the extra full fare. At least under this scenario they'll get getting one fare instead of zero.