Cross pollination between your two citrus is the likely cause of the seediness. When your trees reach maturity and have lots of blossoms, that cross is more likely. I don't know when the tangelo normally flowers, but perhaps, your blossom window have shifted somewhat to make them more coincide? You don't say where you are and if these are pots or in ground (though irrigation would probably be in ground). Artificial lighting can shift the flowering cycle.

They are both flowering at the same time and are near each other in the yard. I live in Hawaii. The lemon had been sickly but is doing better and fruiting more. So maybe it happened because it has more blossoms. is there anything I can do?

You could put a net over one of the trees to keep bees/other pollinators from getting to it.