two LTM4623 in parallel gives ripple?

We put two LTM4623 in parallel and are getting a persistent ~30kHz ripple of ~25mV out.  Any ideas why?  We set to 3.9V and use external sync at 2MHz.  We feel we followed the data sheet design & layout instructions quite closely.  We're gonna try piling on Cin & Cout.  Is it worth trying other things we see on the 2171A Demo Bd:  2.2uF on IntVcc; a RC LPF from Vin to SVin; & a cap parallel to the Feedback resistor?  Thanks!

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