Double rhododendrons are rather few in number, since you want a comparatively low plant anyway - and these are classic Japanese garden plants perfectly in character with Japanese cherries - a double evergreen azalea would probably be the way to go. There are many kinds of these. Possibly one like 'Caroline Gable' would fit the bill. You may have to look around at flowering time to see what's out when the cherry is at its peak. Be aware that potted stock overwintered under cover in nurseries may be flowering at quite a different time than it would on your site. Multiple possible candidates may have already flowered and gone over, that will actually bloom later once planted out on local properties. (Of course, many of them flower before now anyway; in my Zone (8) the season is mostly March-May(June)). If you saw a suitable kind in a nearby garden that would be perfect. Someone at an independent nursery or display garden may be able to give you a name for a flowering specimen cut from a planting in your vicinity if you see one that seems to have potential but do not know it yourself.

Since the peak bloom of azaleas and cherries is so brief you really do need to identify an azalea that will come out at almost exactly the same time for this scheme to come off.

Note: 'Shimidsuii' or 'Shimidsu (-zakura)' is regarded as a synomym of 'Shogetsu'

It turns out I couldn't find a Shogetsu and the consensus here seems to be that the tree may not be hardy enough for zone 5.

I ended up choosing a Kwanzan cherry instead.

So far the best match in bloom appears to be the rhododendro 'Yaku Prince' though it probably flowers slightly later than the tree does.

Any comments on this choice?

Thanks!