Okay, so the reason the first gate opens is due to a reaction to the pendant, correct? Marle has the pendant around her neck, and because she's not really paying attention, she gets sucked into the gate while dropping the pendant. Okay, good so far.
Crono grabs the pendant, and holds onto it, allowing him to use the gate. He still has the pendant as he goes through the gate.
So, how the heck did Lucca "follow"? She has no access to the Pendant when the Gate sending Crono back closes. So, how does she activate the gate?
The only possibly explanation I can think of is that when Marle dropped the pendant, it chipped and Lucca used the chip to create the gate key.