Never connect a torch to acetylene and oxygen tanks without installing a set of flashback arrestors in the hoses at the regulators.
Without these safety devices, it is possible a flash can occur within the hose, and ultimately follow back into the tanks, causing an explosion.
This would be an extremely rare occurrence, but flashback arrestors will make it a "never" occurrence.
Check valves are only a first line of defense and may not ultimately stop a flashback.
Only the flashback arrestors will.
1xAcetylene Flashback Arrestor
1xOxygen Flashback Arrestor
1xAcetylene Check Valve
1xOxygen Check Valve