How to insert caption for multiple figures side by side

You can create a drawing canvas (it's in menu Insert -> Shapes -> New drawing canvas (it is at the very bottom)), put both your drawings in it and put your caption for the canvas.

I would suggest that you use a borderless table with two columns and two rows. You can paste your two pictures in the two cells of the first row, and then add the corresponding a) and b) captions below each figure on the next row.