Some sealers are water-based, others are solvent-based. Mixing the two, maybe one atop the other still accomplishes the goal. But choosing two different solvent-based products could result in unsightly "lifting" of the previous solvent-based product. Solvent-based sealers will generally cut into previous applications of solvent-based products with no harm done. But it is possible one solvent-based sealer would not be compatible with another solvent-based product. The best thing to do would be to test a small area to be sure the products will work together.
Like I said:
Most all the sealers on the market I know of are of good quality and work nicely together.
Keep in mind that sealers are not usually in fact 100% sealers. Stains could still occur. Sealers simply allow enough time to clean up a spill.