I'm not sure what type of trouble I me be getting myself into with the dynamic load a boy swinging (actually boys) will create.
I am concerned that swinging could pull the lag bolt right out of the tree and I wasn't sure if drilling a hole fully though the trees (10" diameter) and putting a support on the other side was a good idea or not since the trees are still growing.
Also, we do have high winds from time to time (but not often) so there could be some relative motion between the trunks.
Keep talking and you wil realize that permanently mounting anything to a living, still growing object is NOT a good idea. As you seem to be speculating, anything could, and will likely go wrong. Growth patterns will not be exactly the same between the two trees even though they're side by side. Soon the growth of the tree will swallow the bolt or the reaction of the metal to the sap and constant moisture will cause the bolt to rust and fail.
Tree swings are meant to be tied loose (with room for the tree to grow), on a limb that is far enough away from the trunk to swing freely, and with rope that doesn't cut into the supporting limb. Nylon twist will strip the bark off and leave the limb exposed to infestation and will cut short the life of the limb. A soft braid is better for the tree and the larger the diameter, the better.
A tree grows by forming a cellular network of tunnels every year as the moisture from the ground is absorbed through the roots and makes it's way up the tree just under the bark. The more moisture and sunlight, the larger the cellular structure. Then winter comes and the structure hardens, the tree kind of dries out and becomes dormant. Then it all starts over again the next spring. A rope too tight, a puncture wound, a bolt through, any of these can disturb the life and health of the tree.
Hardwood trees are better for swings, Oak, Hickory, Walnut, Pecan.
Softwood trees are likely to be damaged by having anything attached.
Hemlock is a softwood, stronger than Spruce, lighter and much weaker than Yellow Pine, which is also a softwood. It's more comparable to Fir, another softwood.
Yadda yadda yadda, they're your trees. Do what you want. If you have plenty of trees to keep you happy, enjoy what you can from these.