Do you think I can use 2 of 6V LED in the row with 12V.

I Would not recommend it. If the two light assemblies are exactly the same voltage ... They it will work. Else it won't.

now there is a device called a DC-DC converter. this takes 12vdc and depending on the type can make it most anything you need... In this case you need a "Buck" type unit makes six out of 12.

or the proper resistor. but the DC-DC Converter...... Better. A few dollars at All Electronics dot com.