I used put my finger halfway to the object, look at my finger until the crossed images in the background converge, then remove my finger and let my eyes stay in the same spot.
> until the crossed images in the background converge
that's the issue! like an OP i never was able to "converge" the images. for me it is phisically impossible: they are too far apart and never come closer. maybe because i have an astigmatism.
Now I can do it without the finger.