Sounds like you could do either way in cool weather. Of course, if you divide it into separate plants, it will take a while for them to grow back to the same width-height as the one you now have.

It should not die back in winter regularly; you need temps to go down a lot below freezing to kill the stems; or you can have die back if the plant breaks dormancy in mild winters, just as a very cold front arrives. On really, very mild Fall and Winters, they may keep some foliage and loose it when leaf out starts. The further south one is, the later they tend to go dormant and loose the leaves. Mine loose leaves around December.

But they usually loose the leaves in winter. I get leaf out in winter or in Spring, depending on whether the winter is mild.

And the brown out blooms stay on for a while. You can deadhead the blooms in the Fall if you prefer. I let Mother Nature take care of them. Usually.

Hint: a lot of the growers of these plants "cheat" by starting four different cuttings in one pot. There's really no wrong way to divide a hydrangea as long as you have some stems and associated roots. Well placed hand trowel slits and you can cut it just like a pizza.

No telling exactly how H macrophyllas will behave in your zone 10. I guess they go through dormant periods and rejuvenate. Might want to try searching the Net for care tips. Normally H macrophylla aren't recommended for zones higher than 9, but doesn't make them impossible to grow, either.

Contrary to common belief, shrubs do not lend themselves to division in same manner as do herbaceous perennials. Shrubs generally have only a single stem/growth point from the root crown, whereas perennials have multiple growth points. Having said that, caning shrubs - like hydrangeas - can be divided if one does it cautiously and with the understanding that you do need multiple stems to create a successful division.

The plant in the OP's photo is really multiple plants and and they can be separated and planted individually, although the end of May is NOT when I'd approach doing this......later in early fall would be a much better time.

My sister used to live just south of Laguna Beach a full zone 10 climate and she had numerous hydrangeas in her garden. They never go completely dormant or lose all their foliage in winter and may even produce a few flowers but growth is very slow compared to the more "normal" spring through fall growth season. And that also seems to be one area where pruning recommendations can safely be ignored.

btw, the USDA hardiness zones have absolutely nothing to how well a plant will do in higher zones. They are ONLY a reflection of how much winter cold a plant is likely to tolerate. Since macs are native to Japan and China, both of which have zones where hydrangeas grow naturally well above zone 9, there is no reason they can't grow well in warmer, even frost fee climates. There are numerous reports of these plants thriving in Central America, Puerto Rico, the Philippines and Hawaii.

I would gently pull them apart and repot in individually. Fall would be a better time and would give them a good start before dividing. They are really young and easier to care for just as they are. I would also keep them out of sun except for early morning - before 9am - sun. Watch so they don't dry out and make sure they drain after each watering. Be warned that they may change color when repotted - it will depend on the soil that you use. There are tons of discussions on soil and color of hydrangeas in this forum. Check them out. Once it starts to cool in the fall - divide and again keep moist in their new homes. They may go dormant - which is pretty scary to see the first time especially if you have divided them, but they will set up new buds and blossoms in the spring. The blossoms will most likely be smaller because these have been forced by the grower for market. In a couple of years they will catch up and blooms will be just as big as these.