Came across this thread. If people are still interested:
Use Irfanview>Thumbnail View (Press 'T')>Select the two photos you want side by side (Use Ctrl-Key)>Then: File>Create Contact Sheet From Selected Files..>Set Columns to 1 and Rows to 2>Set size of final image and space in between and whatever else you want to tweak>Press Create>Presto!
Let me add something that I didn't mention explicitly before. I was a bit 'succinct'
Because this Irfanview function is originally intended to make 'contact sheets' and resizes the images accordingly you have to calculate the size of the resulting image to twice the width of the individual photo and set the height to the original height (plus any borders and space between you might want to use otherwise set these to zero) and set it accordingly. Then Irfanview doesn't (have to) resize them and although I cannot know the internal machinations of Irfanview I assume it doesn't process the images when it doesn't have to resize them.
It does the trick though.