I hope you don't mind me replying in English instead of Traditional Chinese, but I can type faster and can write all the words in English.

Anyway, the "Hidden Combintaion" is generally accepted as the combination between the Horse and the Pig (The Horse's Yin Fire and Yin Earth combine with the Pig's Yang Water and Yang Wood).

The other "Hidden Combinations" are reportedly the Rooster and Tiger; the Rat and Snake; Rabbit and Monkey. Although normally you will only see the most common two being talked about; Horse and Pig, and Rabbit and Monkey.

"Obvious Combination" I believe is simply another name for the normal Six Combinations that are in use.

As to which one is stronger, there are conflicting views on this. Some would say they are of equal strength, but one type being obvious, and the other being hidden. Whilst there are others that say that the hidden is stronger than the obvious types. And of course, those that say the obvious types are more stronger than the hidden types.

Generally, I run the rule that both are as strong as each other. But that is myself. Hope this is of help for you until someone more experienced can tell you more.

Regards,
J1NG