different threshold value of 2 inverters
Switching threshold means inverter is working when the input more than threshold (kind a comparator) . So In your circuit 1.035v inverter's duty cycle is %43.5 other's duty cycle is %83.If you want %50 duty cycle you should chose a inverter with 0.9V switching threshold.Also if you connect these inverters parallel they short circuit each other because of phase difference (one is positive when the other negative (or ground))