I wouldn't assume they divorced, although it's possible - but there are plenty of known examples of "double marriages" for no apparent reason. You need to get both certificates to see how they are described (bachelor/spinster, previous marriage dissolved etc.).
I have a similar case at the moment where a couple married (as widower/widow) in 1944 and then married again in 1955 (both now showing as divorced). The truth was they both lied on their first marriage and their partners were very much alive.
Once they actually got divorced from their first partners they married again ( with one witness appearing on both marriages and also the same registrar). On the second marriage the bride is listed under the same surname as her husband ( with "formerly known as" for her previous name) which is correct because she had been living as his wife for 11 years and so that was the name she was known by at the time.
