The split below the soil line is not into 2 trunks, those are 2 major thick roots.
I am not familiar with growing in a mild climate like Portland OR, in my area this time of year (late autumn, early winter is the exact wrong time of year to be disturbing the roots of a tree.
If your winters are frost free you can repot. If this tree is going to be exposed to freezing weather, do not disturb the roots at this time of year. This is a hardy outdoor tree, do not try to grow it indoors.
When you repot (now or in spring) I would remove one of those heavy roots. Pot the root you remove up separately with the cut end slightly above soil level. THis will be a root cutting, and will likely sprout new beds at the the cut point if the root cutting gets good light.
I would step wise, in successive years, try to reduce the depth of the root system on the 'Texas Scarlet'. It is a nice variety of flowering quince. It works reasonably well as bonsai. Eventually you want it to fit into a shallow bonsai pot.