Question

What happens to cd spectra when equimolar concentration of l and d isomers are in the sample?

Answer 1

If they are the same amino acids in the same order, you would not see anything, since the CD spectra are mirror images with respect to the wavelength axis. There are a few papers claiming that a fundamental asymmetry in nature (parity violation of the electro-weak nuclear force) can have chemical effects and may be responsible for the homochirality we observe in proteins I find that rather (well, highly, actually) doubtful.

In the attached paper (which is about the above effect, maybe you like the idea), CD spectra are shown related to your question in figure 1, which shows that if you add them up (as in a 50/50 mixture) there would be nothing left. Maybe the section Residual CD signal in poly- DL amino acids (p. 337) is of interest to you