if I had a 1M spread option. Would you say that was 1m notional (for IM purposes) or 1m pay + 1m rec i.e. 2m notional?

Assume I have a 1Mspread option.

Would you say that was 1M notional (for IM purposes) or 1M pay + 1M rec i.e. 2M notional?

Since you own a spread option, you are altering the payoff of a single 1M underlying. Therefore, the notional would be 1M. Personally, as long as the slope of the payoff is bounded by -1 <= slope <= 1, I would say the notional was 1M.

For the purpose of calculating initial margin (I assume that is what IM refers to) a clearing house would do a calculation that effectively takes into account the notional amounts of both underlyings, each of their marginal probability distributions (of returns) and their correlation, or if the statistics require it, also the characteristics of the copula joining their marginal distributions.

So there is no simple answer to the question, as the margin will not look something like notional * volatility * constant as it might approximately be for a futures contract. I can however sympathize with the desire for such a simple answer.

This question is frequently asked of risk departments in clearing houses.