I don't see them being that much different that they won't still be within a reasonable tolerance. HVAC guys parallel control transformers when "twinning" units sometimes, I think there is more risk of unequal division issues with something like that.I wouldn't trust the coils to be similar enough to divide the voltage equally.
If they are in series, current is always going to be the same through both coils. Voltage drop across each individual coil can vary if impedances are not the same though.In this scenario what stops the current from choosing one relay over the other? I see a potential imbalance of load on one of the relays. In the case of this imbalance you might burn out one of the relays... This issue comes about because of the possible difference in coil resistance...this can not be guaranteed to match perfectly from relay to relay even of the same p#. Just my 2 cents, Thanks!