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I have not heard of syphilis passing this way.
It is easy for the bacteria that causes syphilis to pass to another person when there is direct skin to skin contact with a moist syphilis sore or mucous patch, but since the bacteria does not survive well when it is outside of the body we do not see syphilis passing from underwear.
The only way I could see syphilis being passed with sharing would be if a sex toy was shared straight away or say if someone wiped a syphilis sore on a damp warm towel and then someone rubbed that towel against their skin straight away.
Let us know if you have any further questions or concerns.
This answer was posted on May 8, 2018