Several of my customers have had a similar configuration. Since there is no measurable signal at the second panel, that could be due to either two separate long runs from a common point or a serious "signal sucker" at the second panel.
The first thing to do is to check whether you have some devices powered by the second panel that are severely loading down the signal. If so, you should isolate them with appropriate X10 filters.
If that doesn't solve the problem, you could add two X10 signal couplers between the panels. This can be done with a 3-conductor Romex feed from one panel to the other, routed as direct as possible. Then add a pair of XPCP's at the second panel, one on each phase, to couple the X10 signal into a new 240V breaker at that panel.
Another solution is to boost your signal level with the XTB-IIR at the first panel so there is adequate signal left after making the round trip through the utility feeds. One of my customers is controlling X10 devices in a second home sharing the same utility transformer.
Jeff