I have a car seat that has a clip that is supposed to have one clip on the ring on the seat and one clip on the harness. But the lead is really long, and when I got Somi I ran the lead through the D-ring on the seat and clipped each end to their harnesses. [this is for the short time my babies got along that is..

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So if the lead is attached with a D-ring time thing, you could run the lead through there and have one on each end of it. And if they pull each other back and forth, you can just knot it around the D-ring.
Good luck on traveling with all your babies!