The filters (assumed to be bandpass) should be a good match to 50 Ohms in the passband. Of course, outside the passband, they are highly reflective. That is why they are filters.
You have a length of coax between the two filters. Out of band, this length of coax has very high reflection terminations on both ends. This makes the length of coax into a resonator.
I think it is likely that the extra S21 peaks you are seeing are due to this coax resonator. To test this hypothesis, simply change the length of the coax between the two filters. The extra S21 peaks should move accordingly. To move the extra resonances much higher in frequency (where they might not cause problems, depending on your stop band rejection requirements), just make the coax as short as possible. If the shortest possible coax is not short enough, you need to design a single filter that will meet your requirements.
One other alternative is to put a circulator between the two filters. Then the out of band reflection from the second filter is absorbed. A circulator will increase loss, size, mass, and cost. However, if you have a circulator handy, it is quick and easy to do.