Just for clarification, open jaw means that you have a break of journey between two fare components. It is only possible if the fares for each component are combinable on a half round trip basis with each other, and this is stated in the Fare Rules/Tariff Conditions. If not then it's two one way fares which is usually more.
By (over-simplified) illustration using fictitious figures and assumptions:
LON - MAD - LON weekend excursion fare is £400
LON - BCN - LON midweek excursion fare is £300
The two fare types are combinable so to form an open jaw journey :
LON - MAD (surface) BCN - LON the fare would be 200 + 150 = 350.
That journey would be a single open jaw
A double open jaw would be for example LON - MAD surface BCN - GLA.
When two fares are combined in this way, the conditions of the more restrictive generally apply to the whole journey.
Of course none of this applies with the LOCOs as each segment is a stand alone.