Yes, your two-shot composite can work, but the easiest believable result is usually to create a real large, diffused light source where the window is.

The key is that window light is broad and soft. Instead of aiming bare speedlights at the fake window, use a large diffused source behind or in place of the window area if possible. Community suggestions included building a frame with actual openings and tracing paper, or using a vertical panel of fluorescent tubes behind diffusion to mimic window light.

A bounced flash may work for soft side light, but it depends on distance and space. If you want the light to look directional, you need enough room so the light rays appear more parallel; otherwise it can look obviously artificial.

If the fake window is meant to act as strong backlight behind a subject, it becomes much harder to make convincing in-camera, and compositing separate shots may still be the most practical option.

So: for the most natural look, use a large diffused source shaped like a window; for difficult backlit scenes, expect to composite.