AWWW - you must feel so pleased!
It sometimes takes a long while for piggies to cotton on, and I had one who eventually learned how, but preferred NOT to! Just give a bit more watery veg (cucumber, pepper, celery, lettuce, tomato etc.), during the time one of them is not drinking, and they will be fine!
Once one of them has figured it out, the "I want what you have" reflex usually means that the other will copy it in the not too distant future!