I would still stick with a single string and until you really nail it, then I would move on. If you still have trouble playing single string licks and syncing with them, or they are difficult like you indicated, I wouldn’t move on to the two string fragments, you may just have more trouble with those even if they are shorter fragments. I also wouldn’t worry about getting anything “up to speed”, I would worry about them sounding correct and locked in first.
It’s not the speed that makes any lick sound impressive, speed on its own isn’t hard to achieve-it’s how rhythmically locked in it is, or how it’s timed, that does, and is harder to achieve.