Think you can't use a twin needle in a side-loading-bobbin machine? Necchi had this figured out back in the 1950s!
The needle, of course, has to be inserted "sideways" to engage with the bobbin thread.
From the front, the two needles are lined up, one behind the other.
Of course, the throat plate and the presser foot must have openings to accommodate the length of that sideways-oriented twin needle. Necchi supplied the twin-needle throat plate and presser foot with their Mira models, but of course they also fit the Novas. Necchi twin needles came in a small plastic bottle, shown above. The bobbin case with the small hole in the "finger" is also required for this technique; simply thread the thread through the hole before inserting in the machine.
I didn't play a lot with this, and the fabric I had handy was too thin for the job... but I did discover that different shadow-type effects can be had by changing the width of the zigzag. The four lines of stitching at top show this; increasing the stitch width causes the two colors of thread to separate until they are a series of Xs. Underneath, the bobbin thread moves at right-angles, rather than the zigzag that is characteristic of twin-needling with a front-loading bobbin machine.
The wavy line in the middle was done using a straight stitch and moving the fabric; the second color overlaps on the straight-aways, and emerges in the curves.
Finally, I tried freemotion stitching with a darning foot, and it works! My test sample is far from lovely, but the possibilities are there!