i have a Superior and a Toka. They were both reccomended as pollinators for each other. The local nurserys around here (and from what i have read) tell me that the Japanese varieties that we have are self-pollinating but do better with cross-pollination. I planted both of mine last spring and they both grew great but i missed pollination so got no plums. So i have no personal exeriance with fruit on my trees yet.

ramble, there are pure Japanese plums that are originally bred in California or Japan. Breeders in Northern states crossed native American plums with Japanese to introduce cold hardiness. Superior and Toka and many others are great American hybrid plums. These American hybrids pollinate most other plums very well due to their wild plum parentage. Toka is said to be the best hybrid pollinator. But, some pure Japanese crosses don't accept American hybrid pollinators. Metheley considered a Japanese type, has wild lineage and is a great pollinator. Santa Rosa a Japanese type plum bred in California long ago is actually a hybrid with wild plum lineage and a great pollinator. Learning grafting is easier than trying to figure out why pollination works.

Here is a link that might be useful: Plums of North America

I would guess you will be OK but if not just graft a limb of something else in a few years - plums are easy to graft. I believe both of those plums are hybrids which increases the odds they will pollinate each other.

Scott

Where can you buy Japanese plum limbs to graft?

Also, I guess you have to graft when dormant?