Grafit is a program that is well-suited to this purpose. You will have to type in the equations yourself, though.

http://www.erithacus.com/grafit/

You can try to determine the initial reaction rate by keeping the concentration of one substrate saturating and fixed and varying the concentration of other substrate at a time.

Then plot Lineweaver Burk Plot for both substrates.

I had the same problem myself. Prism allows that calculation, but requires a highly complicated mathematical procedure to circumvent the problem that it actually can't handle curve fitting with several independent parameters. Because of the mathematical conversion procedure, real-world data (with some real scattering) usually don't give you any results, and you get mostly a "does not converge" answer.

Therefore, I used to calculate these data with an old MS/DOS program "Leonora" written by Cornish-Bowden (Analysis of emzyme kinetic data", Oxford University press 1995). We still have an old computer in a dark dusty corner where this program runs, just have to note down the results by hand and type them into where you want them. The book is sold out, but it is available sometimes in online old-book sales (just check whether it actually still comes with the attached floppy disc containing the program). The copy in the library usually lacks the disc and is therefore useless, since the book is basically just the instruction how to work with the program.

Alternatively, I also found a website you could use for the purpose, but I did not check for some time whether it still works. I cited in in one of our recent papers (Weidenweber et al. FEBS J 289, 5599-5616.

Hello Kirk Hevener Did this link on the GraphPad website not help you?

https://www.graphpad.com/guides/prism/latest/curve-fitting/reg_substrate_inhibition.htm

With the old software, you just get stuck local minimum and you don't get the global.

I have described in the "New insights and tools for the elucidation of lipase catalyzed esterification reaction mechanism in n-hexane: The synthesis of ethyl butyrate" how to properly do the kinetics with surface fitting in originpro software