If you activate STP this is IMHO going to work as intended. You will utilize only one link at a time, though. Don't connect both ports without STP - this will create a loop and bring down your network as
@jgreco correctly stated.
I don't know how to do this in Linux/TrueNAS SCALE, sorry. I could tell you for CORE.
@jgreco @morganL You can have failover - although not aggregation - with just STP, even connect to different switches without multi-chassis LACP etc. I do agree that you should know what you are doing. Don't let your TrueNAS accidentally become the root bridge, for example. I would not do it with unmanaged switches where I cannot checkt the state of STP. With an all IOS environment - why not? We have run a hub-and-spoke switch topology with STP for years.