Welcome to the Forum.  I'll be please to comment.  You are correct that in North America and much of Western Europe rates of secondary syphilis have increased among men who have sex with other men and that syphilis can occur at any site of sexual contact.  While VERY rare, I have seen syphilitic lesions both on the finger and on the lip/mouth. Further, because syphilis can be so varied in the way it presents, it is appropriate the have a high index of suspicion.  Having said that, even acknowledging the facts I mention above, the likelihood that the lesions are syphilis is low and you can quickly determine whether or not either of the lesions are syphilis with a blood test.  A few more background facts/comments below:

1.  I doubt that both lesions are syphilis although perhaps one of the other could be although this is statistically unlikely.  Syphilis lesions develop, on average, about 21 days following direct sexual contact with a lesion.  Initial (primary) lesions are virtually always solitary and I have never seen or heard of a situation in which someone developed two primary lesions at different places. 

2.  Because syphilis lesions result from direct contact with a lesion, it would be possible or even probably that you might have noticed a lesion at the time of contact with your prior partners.  If not, the likelihood that either of the lesions is syphilis is a bit lower.

3.  You did not say but if you know your partners, asking them about their status or possible lesions might be helpful.

4.  As you suggest, following development of a papule, syphilitic lesions typically go on to ulcerate in a few (2-5) days.  If the lesions become pustular (i.e. like pimples) they are unlikely to be syphilis.

5.  The contacts you describe could also be early herpes although if either lesion was herpes, I would have expected them to become shallow ulcers within a day or two of appearance.  While it would be unusual in most settings for testing your lesions for syphilis to be readily available, testing for herpes is widely available and if I were seeing you in clinic, given your concern, I might well test each of the lesions for herpes using a PCR test (FYI, many health care providers mistakenly think that herpes can only be detected in ulcers or fluid filled vesicle but this is incorrect). 

6.  If the lesions have been present for more than 3 or 4 days as you suggest, it is quite likely that if they were due to syphilis a syphilis blood test would be positive at this time.  A negative syphilis blood test would strongly favor that these lesions are not syphilis.

I hope these comments are helpful.  Depending on how things evolve and your level of concern, you may want to get a syphilis blood test and perhaps herpes PCR testing of your lesions.  Unless you are planning to have sex with someone in the next day or two, this is not emergent and waiting to seek further evaluation (and watching the lesions for another day or two over the weekend) should not be a danger to you or to others.  EWH