There are multiple reason why wood splits, most probably not because water is being absorbed, but rather it is drying out at a different rate in the wood. The reason that a sealer might help is because the end-grain of the wood drys out much faster than the wood in the center of the beam and the sealer on the end-grain tends to slow the process down. Also, the faster the wood is dried might increase this effect, as it takes a long time for the moister to escape the center of a 6" beam and any attempt to speed the process up will increase the risk of splitting.
You might also be dealing with the forces of the wood fibers. Unless the grain is very straight, when the wood drys, all of the fibers pull in different directions. This is unavoidable but the slowness and completeness of the drying might make it better (or worse). If your beams have a lot of curve in the grain you might also experience bending or warping as they dry out. This is where the term, "wood moves" comes from.
If you want to minimize the chance of splitting, my recommendation is to paint the end-grain and put them (uncovered) in a dry, cool, dark place for 6-12 months. This way the wood will have a chance to dry out as evenly as possible, noting that there may still be cracks.