Full disclosure: much of this Answer is going to be hand-waving, because no one (especially strangers on the internet) really knows how a particular chunk of wood will react to unknown environments over time. There is experience that can be gained that will suggest how these materials change over time, but this will only go so far. This is not to dissuade you from your idea! Just be prepared to change your approach as you learn.
It looks like you are interested in working these blocks to the desired shape green, and then letting them check and split a little before stabilizing them. This is not an unknown process, given the prevalence of "live-edge" carpentry these days.
Green wood can be easier to work with, as it is soft and forgiving. So, there is no reason you couldn't work these pieces to the desired shape while green. You could even soak the pieces in water to slow the drying while you work on them. Just change the water regularly.
Remember that the finished product may end up a different shape, and a little smaller than where it starts. So factor this into your design. Or be prepared to be surprised.
Ok, so now you have two pieces of wood shaped to some desired profile. Now you want to creatively "cure" it. This means time; perhaps even weeks. Let's assume you have weeks to wait. It may be better to control the drying so it is slower. I'm not sure how you might easily do this, but I've heard of turners (etc.) placing pieces in plastic bags to retard the drying process. This might be a place for more research or experimentation.
As they dry, they will check and split, and maybe they will split all the way through. Since wood is a natural product it is hard to predict how it will move as it dries. But, assuming it dries into some profile you like, now you can consider how to stabilize the pieces.
There are many wood stabilizers out there, and some folks even make it themselves with stuff like polyethylene glycol. Again, this is a place for some research. Pay attention to what carvers and turners do in this regard, as they tend to work less with dimension lumber, and more with larger hunks of tree. But, I'm assuming there are also tough poly and epoxy style finishes that could seep into cracks and cell walls in order to stabilize and seal the pieces. The idea is to let them dry just enough so you can stabilize them. This is partly science with respect to moisture content (you might want to buy or borrow a moisture meter) and partly art with respect to how it looks over time as it dries.
One caveat that I can think of is that as they dry they are going to get lighter and lighter. Maybe to the point that they don't really work as book ends anymore. A good amount of the mass of most wood is moisture, and many species will get very light as they dry. I think species like oak and walnut have more mass when dry, and my assumption is that you will want to stay with dense true hardwoods for this project, as expensive as that can be to make mistakes with.
And an editorial comment:
Given there is (and of course there is) a Japanese art of letting wood naturally age and crack over time in order to put it back together with specialized carpentry, maybe this sort of approach might work? If this stuff start to fall apart, maybe figure out how you can use butterfly joinery etc. to put it back together in an artful manner. From this perspective there is a certain symmetry in observing a well-aged and oft-repaired piece of art as a parallel to married life.