Is 14.2 acceptable for an electric baseboard?
The limits on wire size are based on amps, not watts, so we would need to know the amps the heaters require before determining the wiring requirements. 14 gauge wire is good for a 15 amp breaker, but for a continuous load you are supposed to derate to 80%, which is 12 amps. On a 120V circuit that would be 1450 watts and on a 240V circuit that would be 2900 W.

Also, how and why would you wire these in series? Virtually nothing in household wiring should be in series (except for switches). A simple resistive load like an electric heater would probably work if wired in series but it would dramatically reduce the heat output of both and is certainly a violation of the manufacturer's installation procedure.