When you take them outside do you just amble around the yard? If so, I'd try to get in some brisk walking. Nothing gets a dog to "go" like a good brisk walk. And are they eating on a regular schedule or are they free fed? Most dogs who have scheduled meals usually eventually develop somewhat regular pooping times, so you get to know when they need to go.
Ditto the potty bells suggestion.
It's not uncommon for small breed dogs to take close to a year to get fully house trained. Having two at once makes it even more difficult. I'd say all in all it sounds like you're doing pretty good.