Collective2 does not support spread-order-entry. This is mostly because some of our broker partners do not support it, and to be "broker-agnostic" and to support as many different brokers and users as possible, we generally need to support least-common-denominator order systems.
So to trade spreads you will have to "leg into" the trade, one leg at a time. You'll probably want to leg in to the long side first, so that your short margin requirements are not onerous.