I don't think there's any way around this at the moment. I got used to clicking the left side before clicking the button.
Hmm, actually, you may be able to do it via running Set FOCUS=Left and then running a user command which runs the diffing tool. I suspect (but have not confirmed) that the user command's arguments will be evaluated just before it runs, after the focus has been set.
Even if that works, though, it means the command is in two places which isn't ideal, unless you only have one diff tool. (I've got about 4 diff tools all with Opus buttons, as you can see in my diff toolbar, since all of them have their strengths and serious weaknesses. 
)
By the way, you probably don't want to be using allfilepath in either of your arguments. That would include multiple files on the command line if more than one file was selected in either side.
Also, for when comparing two directories (rather than files) you can use {leftpath} and {rightpath} so that the paths are always the same way around. Unfortunately there isn't the same thing for selected files.