No problem at all, I do this myself.
They can actually even be installed in the same folder, but personally I'd recommend installing them in different folders.
There is no interaction between the two versions at all.
In fact I have programmed a toolbar button on 12.17 to open the displayed page in Opera 24 (the current developer version), so if a site malfunctions in 12.17 I can open it in the later version.
All works fine.
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So how do you accomplish this, installing this button to open page in new opera?
Open standard_toolbar (1).ini in your profile\toolbar folder with notepad.
Find the [Browser Toolbar.content] section.
Add there -
ButtonXX, "[button name]"="Execute program, "[path to opera 15+]\Launcher.exe", "%u", "[button pop-up label]", "[button icon]""
XX is the button position on the toolbar, replace the bits in square brackets with the button name, the path to your Opera 15+ installation, and the pop-up label you want on the button. If you want an icon on the button, you will have to make one and put its name in "button icon", but this isn't essential.