[Autumn]

When you are utilizing any of the "Contract X" skills, your guardian spirit will not be considered inactive regardless of the slot it is in, this also means that you can have two "Single Bonded" youkai active at the same time, there are a number of inconsistencies this can provide, but for the primary purpose I don't believe it is fully intended to have Two "Single Bonded" youkai active given the existence of Instant Guardian, but Four Bonded I could possibly see, here's the specifics:

To Replicate:
1) Gather 3 individual contracts with Contract Tres equipped
2) Bond specifically one of those contracts.
3) Place Guardian Spirit in Slot 4+ (also make sure it is bonded)
4) Purchase and equip the Instant Guardian trait.
5) Check if you gain Single Bonded bonuses (such as Fight As One while installed).

Notably the main bug that I've found from this is you are not allowed to unsummon the Guardian Spirit actually, it also allows you to have a "4th" contract.