Hi Michael, Id like to say right off that your palms are incredible. Showing them in a potted habitat is very worthy to anyone studying growth patterns of the Trachycarpus fortunei.
I've tried to remember about Windmills Ive grown in the ground and that height, they did wait for years before blooming and making seed. Then it was a yearly thing.
I wonder if they are both males or both female? Its hard to know unless they bloomed as recent to remember. It could be they need to be put in the ground. I know, we always worry about an unknown winter that could happen but I think (just my opinion) planting them could get them blooming. If you do plant them, go ahead and get a couple more windmills, 2 or 3 gallon for reserves and to get potted. You could also try not using the lights when the temps are above freezing but I really believe that's not the problem but you could try turning them off in mild weather. Most Windmills here bloomed out by August but Will over in Winston-Salem NC has reported them blooming or throwing out influences as early as March. Hopefully Fred, Kevin, Ken and others will have a good reply. Thanks for sharing Michael.