Merge 2 videos side by side using ffmpeg and hardware acceleration

Question

I know that the most basic syntax to horizontally stack two videos side-by-side is using the following syntax:

ffmpeg -i left.mp4 -i right.mp4 -filter_complex hstack sideByside.mp4

How, if possible, to do this using existing hardware to accelerate the process?

In other word, how to achieve this horizontal stack using h264_vaapi encoder like this:

ffmpeg -i left.mp4 -i right.mp4 -filter_complex hstack -c:v h264_vaapi sideBySide.mp4

thanks in advance