There's no clinical evidence that in vivo there is any added benefit to prescribing the sulfa with trim. Here in the UK doctors only use trimethoprim. You should read this:
https://en.wikipedia.org/wiki/Trimethoprim
"rimethoprim was commonly (since 1969 in the UK) used in a 1:5 combination with
sulfamethoxazole, a
sulfonamide antibiotic, which inhibits an earlier step in the folate synthesis pathway (see diagram below). This combination, also known as
co-trimoxazole, TMP-sulfa, or TMP-SMX, results in an
in vitro synergistic antibacterial effect by inhibiting successive steps in folate synthesis. This claimed benefit was not seen in general clinical use.
[3][4] The combinations use has been declining due to reports of sulfamethoxazole
bone marrow toxicity, resistance and lack of greater efficacy in treating common urine and chest infections,
[5][6][7][8] and
side effects of antibacterial sulfonamides. As a consequence, the use of co-trimoxazole was restricted in 1995
[9] following the availability of trimethoprim (not in combination) in 1980."