It's almost as you describe it: Both injectors are connected to Flexitune INJ4 and only one injector control signal (eg. Cyl 4) from the ECU is connected to ECU4 on Flexitune. This will result in a batch fire connection for cyl 4 and 5. It's also adviced to use a 200 ohms power resistor connected to the ECU to emulate the injector for the fifth cylinder (possible faul codes).
In practice the connection will work fine. Regarding injector fire sequence, it has marginal effect. For example the eraly Volvo five cylinder models has batch fire on all five injectors (completely non-sequential system).
Of course we recommend to use two Flexitune units but conidering the savings on using only one Flexitune the connection above might be considered.