yes, you can two button cell in series. I suggest to use cell like CR2032 this cell is 3V so in series connection we get 3*2=6V so use silicon diode like 1N4007 in series(anode with positive of battery and cathode with circuit positive terminal) to get 6-0.7=5.3Volt. Button cell can not serve as long as pencil cell. ypo can calculate how many hour cell can serve by hour=mAh rating of battery/current consumption of circuit in miliampere. 
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