Welcome to the forum.

I assume both images are A4 size, and you want to print them side by side on A3.

Try this:

Open one image in PhotoPlus and Copy it (Ctrl+C)

Open the second image separately in PhotoPlus
Under Image/Canvas Size, double the width of the Canvas
Paste as new layer (Ctrl+L) - a new layer will be created, and will be selected already.
Use the Move tool (4-way arrows) to reposition the new layer as required.

I hope this helps.

Geoff