Customers of either network have been able to share signal for quite some time now. The idea being, that if you should lose signal on your primary network, if signal is available in your area on your secondary network your phone will automatically switch to it.
The smart signal sharing is designed to make this process seamless, and will prioritise signal from the other network should your current signal begin to fade. The theory being that customers will have better signal more of the time, without the interruptions of having to search for signal on the other network.
Looking ahead, Everything Everywhere are still confident of providing 4G technology before the year is out. Speaking to tracyandmatt.co.uk, the company has indicated they are moving in the right direction to roll out the upgrade this year. The only potential sticking point, is the final go ahead from industry watchdog, Ofcom. Ofcom have already preliminarily approved Everything Everywhere's plans for a 4G network though, so there shouldn't be anything to worry about. If the OK isn't given, the necessary hardware and software will still be installed as planned, so as to go live as soon as possible.
Source: Tracy and Matt