In this pic from the original thread they seem to be sharing the oscillator.http://www.modxhost.com/ass2.jpg
Maybe the solution is one is connected normally, and the other just get its input driven.
Paragraph 8.4 seems to suggest that would be viable:
"Full Swing Crystal Oscillator
Pins XTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier which can be
configured for use as an On-chip Oscillator, as shown in Figure 8-2 on page 29. Either a quartz
crystal or a ceramic resonator may be used.
This Crystal Oscillator is a full swing oscillator, with rail-to-rail swing on the XTAL2 output. This is
useful for driving other clock inputs and in noisy environments. The current consumption is
higher than the ”Low Power Crystal Oscillator” on page 28. Note that the Full Swing Crystal
Oscillator will only operate for VCC = 2.7 - 5.5 volts."