Sorry to dampen your enthusiasm for the twin trunk trident. I can't see the space between the trunks well from that angle but I do not think this is suitable for twin trunk long term. As trunks thicken (and they do, even if it is slowly) the space between the trunks disappears so the fork effectively moves up the trunk. The narrower the space is, the quicker this happens and the further up the trunk the fork gets.
Have fun with this by all means if you wish but in 10 years, when you have spent lots of time and effort, this will probably not still be twin trunk.
I think the problem with budding on the Japanese maple is that you are too hesitant. Most trees try to bud close to the top of the trunk. If you cut high you get high buds and that appears to be the case with both these trees. If you really want buds further down the trunk you need to go for broke and cut low. It is esasier and faster to regrow a new trunk than to get low buds to sprout on a tall trunk.
The other option to get lower branches is to graft them. Approach graft or thrad grafts are reasonably simple and reliable and you can use long branches from the top of the same tree so the new branches will be the same as the rest of the tree.
At the moment, neither tree has much taper. Taper is important in bonsai - thick at the base and thinner near the top. Cutting low and regrowing the trunk will give you better taper. Cutting and regrowing several times over a number of years will give even better taper in the trunks.