1. Wouldn't recommend it as a general principle. Flexis are often not top quality, have restricted bores, and can cause a real mess if they leak.
2. Hard to tell from your photo, but they don't look like the same size. Upper one looks like the compression connection for 15 mm hard pipe, Lower looks like a 3/4" BSP female.
3. Even if they are the same height, the sharp edge of the compression angle in the upper can cut into the rubber sealing washer of the lower, possibly causing the joint to leak.