You can certainly take a 5-ASA along with sulfazine; I do it and have done so for many years now. Several people here and on the UC forum are also following this path.
It is true that sulfasalazine breaks down into multiple components in the gut, one of them being mesalamine. The story behind using sulfazine (same as sulfasalazine) for enteritis arthritis is a good one.
As told to me by my GI, sulfazine was one of the earliest nonsteroidal meds approved for rheumatoid arthritis. At that time ('50s) there was not a good understanding of the connection between inflammatory bowel disease and arthritis. Some people who took sulfazine for their joint pain coincidentally noticed an improvement in their gut problems. So scientists were aware that this med treated inflammation in multiple areas without really understanding why.
Eventually sulfazine was not prescribed for rheumatoid arthritis because other, more effective meds came along, and it was difficult for some patients to tolerate. GI doctors were working with mesalamine directly, and didn't use sulfazine because it was difficult for some patients to tolerate. A few years ago, some bright bulb figured out that hey, there are these people who respond to mesalamine for their IBD and also have joint pain - would sulfazine work for them? The regular mesalamines don't touch joint pain, but sulfazine does.
I started using sulfazine around 2008 and I think that was the beginning of the resurgence in interest in this medication. I do not believe scientists understand the mechanism that makes sulfazine effective in treating joint pain associated with IBD.