Talking is definitely the way to go! It can help to come up with some phrases or the like so they can start to recognise them: "It's ok! You're ok!" That kind of thing. I have little songs to go with different activities and interactions with them, and it doesn't take them long to pick up that this one means they're going to get fed, or this one means the cage is getting cleaned.
I've also found that using the piggies against each other works, too. They usually want what the other one has, or the more confident one gives the less confident one more courage to come up and see what the other one is doing. And also when you're handling them and having lap time, having them both out gives them a lot more confidence too. They're not on their own, and can gain comfort from their friend.
And this one sounds weird but: Poo picking! This has worked for me with both sets of piggies. I take everything out of the cage but leave the piggies in, and basically just ignore them (still talking all the while). That way they learn that me being around doesn't mean I'm going to try and interact with them or pick them up, etc. If they want to initiate contact, they can (and usually they like jumping in the dustpan). They can come up and touch my hand, see what I'm doing, etc.
And finally it is just about having patience. Every piggy is different. For example, it took my boys around three months to start wheeking at me for food, but my girls figured out within a week that the bag rustling meant they were going to get food. My girls were also fine with me walking by the cage withing a week whereas my boys used to run and hide (and still do sometimes) the moment I stood up. But my girls are slower to take food from my hand. Arthur and Merlin did almost instantly, but Ava is just starting to (although my hand seems to be tastier), while Minnie just pins my hand down and then runs off.
It really is just a case of letting them do what they want.
