Both titanium and carbon steel can be spot welded. Information on spot welding them to each other is something that is not readily available in the published literature. Using an intermediate filler metal makes joining them potentially easier. This is no longer a spot weld it has become a resistance braze. Depending upon the part size a resistance braze can be a few cycles or over a minute long. The goal is not to heat the whold part, just the braze medium.
Once again published literature does not have any information listed for the braze medium for resistance brazing titanium. Literature using other brazing methods does show that titanium is frequently brazed using Silver/Copper braze alloys. It is likely that one of these would be suitable for resistance brazing. Flux will be required and are generally available for most Silver braze alloys.
Induction, atmosphere furnace, vacuum furnace and torch brazing are used to braze titanium. Literature did not demonstrate resistance brazing being used but it is similar to some of the above and with some research should be adaptable to the process desired.
A place to start would be to contact braze alloy supply companies. The web listed several that offered braze alloys and fluxes for titanium brazing. A few showed videos of brazes being performed.
If resistance brazing equipment is needed many companies are available to assist in this area with new or used equipment.
Reference: ASM Metals Handbook Vol 6 “Welding Brazing and Soldering”
TWI -Thompson Welding Institute
RMWA - Resistance Welding Manual 4th Edition
You Tube: Induction brazing Titanium