Open the Photos app
Select the two photos you want to combine
Tap the Share button
Scroll down and tap Copy Photos
Open the Shortcuts app
Tap the plus icon to create a new shortcut
Tap Add Action
Search for and add Select Photos
Turn off Select Multiple if you want only two photos chosen manually, or leave it on to choose both
Add the Combine Images action
Set the mode to Horizontally
Add the Save to Photo Album action
Run the shortcut and choose the two photos
Open the saved side by side image in Photos