It has been 45 years since I worked with SCRs. I remember you have to connect the trigger wire to 1 of the SCR power terminals then connect them to an AC transformer. When the AC transformer goes, plus, minus, plus, minus, plus, minus, plus, minus, SCR1 and ACR2 with go, on. off, on, off, on, off. They work like diodes connected like this. You can apply power to and SCR and no current flows until you apply power to the trigger wire. Once the SCR is ON it will not turn off until the power if off or the power goes from negative to positive. The only way to do what you want is use reversing relays to swap positive and negative back and forth to the SCRs then the 555 timer can be used to turn the relays ON/OFF at any duty cycle you want from 10% to 90%. One relay with DPDT contacts will work to reverse the DC to each SCR. When SCR1 is positive SCR2 has to be negative, relays switch then SCR1 is negative and SCR2 is positive. This was one of my learning projects in college electronic lap class. As far as electronic goes it was the stone age everything was just starting to switch from vacuum tubes to solid state.