Spot welding stainless steel to titanium is definitely in the world of research. Both materials can be resistance welded. Data is available in AWS C1.1. It does not cover resistance welding the two together.
It is known that both materials are reactive and sensitive to heat. Titanium expulsion can be very exciting.
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A serious shield for protection from expulsion would be in order. Stainless is austenitic and can subject to interface cracking. On the positive side they both heat up and spot weld readily.
Some digging in the research found that using an interlayer of aluminum might not work due to the formation of some undesirable intermetallic phases. Further research found that magnesium might be used in laser welding and copper was being used for friction stir welding. Both might be worth a try as a resistance braze medium if a spot weld cannot be developed using the data from AWS C1.1.
If a brazed joint is acceptable then consider that both titanium and stainless are being brazed to themselves in industry. Braze alloys exist. Contact a brazing alloy supplier and search for a braze alloy that might be suitable to join them together. A resistance welding machine can do the job depending upon the quantity design and engineering specifications.
Reference: AWS C1.1, Recommended Practices for Resistance Welding
RWMA - Resistance Welding Manual 4th Edition