How to choose printer with dual extruder?
As you suggest yourself, ordering test prints of some model is one way to do it. 

3D Hubs and MakeXYZ allows you to get your model printed by hobbyists and small businesses for a fair price. Both sites also allow you to order prints based on printer type, which I believe is what you may be looking for.

On 3D Hubs, visit on of the trend reports, and select the printer you want a sample from. Similarly, on MakeXYZ, search local makers for your desired printer.