This is what I guess you want to do (a picture would have helped lots!):
1. You have boxwood logs cut 30 years ago which have split lengthwise.
2. You want to cut along the split, plane each side to a good surface, glue them together, and then turn a drawknife handle.
Yes?
If so, it might work. Ordinary woodworking glue is what you want.
The risk is that once you remove wood by planing and turning, you will relieve stresses in the wood. If so it will twist and split, almost certainly.
The only way to find out is to try. Once you've rough turned a handle, leave it somewhere which has no humidity control for a week or so, maybe under cover outdoors. If it survives that it should be OK and you can finish your turning.