Can I make a double board at smart prototyping pcb service?
Assuming you are referring to the prototype service by the name in your title, you must use V-grooves rather than drilled holes.  



They will almost double the price on you anyway (8.55 EUR), so I doubt it's worthwhile. 

Always read the conditions and rules carefully, there are often such gotchas in the details. 

If you have a metal shear you can just space them apart, add silk screen markings and shear them when they arrive. The FR4 does tend to dull the shear.