(1)两灯泡的额定电流:I1= P1 U1 = 3W 6V =0.5A,I2= P2 U2 = 6W 6V =1A;两灯泡的电阻:R1= U1 I1 = 6V 0.5A =12Ω,R2= U2 I2 = 6V 1A =6Ω.∵两灯泡串联,∴允许通过两灯泡的最大电流I=I1=0.5A,∴电路两端允许加的最大电压为U=I(R1+R2)=0.5A×(12Ω+6Ω)=9V.(2)电路消耗的总功率P=UI=9V×0.5A=4.5W.故答案为:9;4.5.