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.
You can also make one of the legs "conditional upon" the filling of another leg, if you like. You would use C2's "conditional orders" for this. See related article: Conditional Orders.