The "RMS" rating of the GB18B is quite conservative by today's more marketing oriented standards (that run my engineering personality the wrong way) so I wouldn't expect any issues with this cabinet. It will split the power equally with your other cabinet.
A couple of cautions however... verify that the cabinet and driver have not been damaged by the previous owner, and that it has the original 8 ohm driver (should be GBE-18300-8 ohm). Also, it's possible that the overall acoustic response of the combined cabinets could be uneven at points because there is sure to be a difference in tuning profiles. This is a case where the cabinets were not designed to work together and the resultsay not be ideal. Try them together and see.
Product Development Engineer: Mesa Engineering
Continuing Product Support: Genz Benz