Tridents do handle root work really well, so you should think about cutting off the old underneath roots. Those happen when nurseries up pot the tree and it grows new roots at the surface. If the top set of roots is strong, no reason you will have a problem getting rid of the lower section. You could also do a chop at the same time on one of the trunks. I’d probably do the rootwork get it on a board,
Agree totally.
Someone has planted this tree in the ground but planted a bit too deep. Tridents almost always grow new roots right at ground level. I used this to grow tridents commercially for a while - no need for initial root work, just slap the trunk deep in the grow bed, leave for 2-3 years. Success rate was around 70% good new roots. Then dig and cut the old roots leaving a flat set of new roots and a flared trunk. I've since developed another technique that gives even better results.
Point is, be careful when planting tridents in the ground. Don't put roots too deep if you want to keep those roots.
No problem cutting the lower roots completely. Tridents have no problem with up to 90% root reduction. I usually also cut the lateral roots much shorter on the initial root prune. New roots almost always develop from the cut ends so if you want the roots to ramify closer to the trunk that's where you need to chop them. I typically cut roots around 1 trunk diameter long. With hard root prune like this I often get new roots developing which could save the problem of grafting new roots.
Make sure the roots are totally covered with soil after potting up. The ends MUST not dry out if they are to develop new roots. Cover with enough soil so the entire nebari is covered for the first summer. Plenty of time to show the roots after new roots are growing strong.
I know planting on boards is a popular thing now but I don't find planting on boards helpful. Once deeper roots are cut they don't tend to grow again. Because tridents rely on lateral surface roots they will almost always develop in preference to deep roots so the board is redundant. During board planting trials I noted that any small roots that are missed can thicken under the trunk and lift the nebari off the board anyway.
Chopping the trunks:
Your biggest problem is no taper in either trunk. It will take a few years but chopping low is the only way I know to rectify lack of taper. First trunk chop is typically less than 1/3 planned height of your bonsai so If you have a size in mind that will help make decisions.
Second problem is a cluster of branches on the main trunk around the height of the second trunk. This is common but quickly leads to local swelling and inverse taper. I suspect the trunk already has that happening which means chopping below there is probably the best option.
It's usually better to chop to a strong side branch. Hard to tell with the odd photo angles whether there's any useful existing branches but it appears there's one strong branch with a slight upward angle among that cluster. It's not clear whether that one is upright enough to become the continuation of the trunk or if it emerges in an appropriate spot to use the 2nd trunk as a twin trunk. I'd consider that branch as possible new section of trunk first with a sloping cut behind the branch to help with trunk taper.
If that branch is no good I'd chop just below the cluster of branches and cross fingers.
In either case, also chop the 2nd trunk lower too - probably down to the first node on it.
