It can be detected by requiring everyone who uses single journey tickets to pass through the ticket barrier to activate the ticket, but it may not prevent a passenger running out of a train at a station, getting out and in using two tickets, and running back to the train again, unless the station layout means that it is impossible to physically run from the platform to the ticket barrier and back in the dwell time of the train.
For example, in Hong Kong, tickets are encoded with the ticket value only, and split ticketing always require physically getting off the train, get through the barrier, and return again. For example, most long distance journeys starting at Lo Wu / Lok Ma Chau are cheaper splitting en-route, so in the past commuters generally prepare two Octopus cards, board at the correct carriage such that they can access the gates at platform level in the minimum time, and when the train stops at Sheung Shui, a crowd runs out of the train, touch out using one card and touch in using another card, and run back into the train. This can be done within the dwell time of 20 seconds. (The gate is programmed such that a touched-out card needs to wait for one minute to touch in again, which is too long to get back onto the same train)
If this is done with single journey tickets, it also require passing through gates as all stations are gated.