Problem is they're synonyms... you're going to need to depend on the stories you made not to confuse the two. They will be many more synonyms as you progress in RTK... I had the same problem as you for a long time. You could try using anki (srs program) rather than this site's quiz to learn the kanji, that way you'll see the story each time and you will not risk confusing the two.

In the end, after you finish RTK, it's not very important if you make a mistake with synonyms, as long as you can write the kanji correctly and you do not interchange them in real words. Heisig says himself that after some time you will begin to extract only the general meaning of the kanji, independant of the learning language (keyword begins to vanish, general meaning remains).

Would Japanese word examples help? Like,
肖像 protrait
如し like, the same as