Originally Posted by CliffordK

One option you might consider is a right/left brake reversal. Allow the best brake modulation with the right hand going to the front. Then, inducing a skid with the left rear might not be so bad.

There are several options for combined mechanical brakes including dual cable levers, or cable splitters. You could do cables to hydraullic (TRP/HY/...)

I've thought about symmetrical single actuated hydraulic brakes with respect to trikes (a good place to start your hunt).

The biggest problem with a simple line splitter is matching the master and slave capacity. I had thought maybe a moped master cylinder???

However, a quick search on the web about hydraulic trikes leads one to the the Tektro Auriga Twin.

https://www.bike24.com/p2127047.html

It looks like they aren't particularly cheap, but perhaps less expensive than a few trips to the dentist.