It would be the other way round, the way Presto have currently implemented TTC transfers.
If they took subway A and then bus B on the outbound trip. If they didn't tap onto bus B, then when they returned on bus B, and subway A in less than 2 hours, they would would get charged a $0 transfer when they tap on bus B to return.
What's odd, is that if there trip the other way round, bus A and then subway B; they'll get free entry into the subway for the return trip, under the current and future system. Unless they change how Presto has implemented.
I hope they adopt a simple 2-hour transfer to avoid all this.
And with two devices, they'll lose that revenue less frequently.
Though the Presto devices seem more prone to failure. I think I've maybe in 10 years seen 1 farebox not working. I've seen 2-3 Presto readers on streetcars not working in the last month.