It is a common practice to bolt beams together to make a wider beam, however you DON'T want to laminate the beams one on top of the other as most of your strength comes from the depth of the beam. I would recommend looking at bolting 1" or 2" x12" boards together to make your desired width.
Also with a large beam as you are talking about it may be more feasible to make a composite beam (laminate the board with steel plates, the board provides width for mounting other things while the steel plates supply the strength). The elastic modulus of wood is roughly 12GPa, while steel is 200GPa; meaning that a 0.5"x12"x18' steel beam would have more strength than the 8"x12"x18' beam. The equation that I used was:
Steel width (in) = (12 GPa Wood/200 GPA Steel) * Wood Width (in)
By making the wood narrower and adding steel plates the effective cross section becomes an I beam.
This is more than you probably wanted to know but I thought that I would answer to the best of my ability. You could look more into this on the net
www.engnetbase.com is a good reference site.
Mark