What does it mean - can be made into a double? Sugino Single Speed Arm Set 175mm Black 130mm

In the description it says: RD2 can be made into a double with double bolt sets.
Can I use these crankarms on my 2 front - 8 rear speed road bike?

What other parameters I should research before buying it?
(130mm BCD, 110mm bottom bracket)
You need a JIS bottom bracket (ISO may work, but ideally you'd want JIS) of the appropriate spindle length for the correct chain line on your bike. 

You'd have to buy a set of chainrings for a double and the bolts for a double chainring in order to use it as a double.