Those are pretty rare by now, mostly because they've been superseded by newer technology. There were a number of methods for mounting the additional injectors - I've seen 2 mounted in the side of the intake manifold, or two mounted to the 3000 pipe on the top of the engine. I've also seen 6 extras mounted in the intake runners, for that matter. The electronics to control it is called an HKS AIC, short for Additional Injector Controller.
Back when this was current technology, using larger injectors wasn't a good idea, because at that time minimum pulse width from injectors was significantly larger than today, which means that getting a decent idle was impossible. Injectors are much improved today, so in modern setups we just use bigger injectors.
Here's a pic of that type of setup with the extra injectors in the side of the intake plenum.