In addition to the issues regarding bright/dim commands and extended codes, there are a couple of other problems with the proposed technique.
The TW523 only reads back the second half of the X10 command. So the signal relayed to the second house would only contain the second half of each transmission. While that should still work, it does eliminate the redundancy built into every command to deal with switching transients and other short-term events.
The X10 spec says the TW523 data output should be asserted within 200uS after the zero crossing detect. However, the data input should be asserted within 50uS after the zero crossing detect. So, the data pulse fed to the second TW523 could be 150uS late. If the source of the signal is a controller that relies on the zero crossing, and delay times are near the maximum limit, the extra 150uS will result in the second TW523 not beginning its transmission before the "X10 reception window". There is sufficient margin built into the protocol that this should still work, but there is little room left for timing errors.
You would not want to couple the zero crossing detector outputs between the two TW523 units.
Jeff