The heart chakra cord between twins never switches off. In separation, it can actually become even more noticeable, so what you’re experiencing is completely normal (though @Cassady is right).
When you’re actively thinking about them, you’re tuning into their frequency more intensely. Your conscious attention amplifies the energy flow through that connection, which is why the pain gets stronger. It’s like turning up the volume on something that’s always playing in the background.
The random pain out of nowhere… that’s usually coming from them. Either they’re thinking about you, going through something emotionally intense, or working through their own heart chakra activation. They’re probably not aware that they’re reaching out to you, but the runner is usually more attuned to what this connection is, so they know where the energy is coming from.
Chances are, when you think of them, they get similar experiences, but it really depends on where they are in the awakening process. Often, they’ll try to ignore the sensation or rationalize it away as something else.