Mitch,

questions like these require so much personal opinion many (if not most) are afraid to tackle them. I’ll try:

“1) Is it better to a) graft both varieties onto the rootstock at the same time? or b) start with one, then graft the second a year or two (or longer) later?”

1) It’s more common to have two or more varieties bud grafted to a rootstock or selected variety at ‘the same time.’ If you’re attempting to establish these trees on a dormant ‘one year’ rootstock by ‘bench grafting,’ it’s not (reasonably) possible to graft two at the same time, only one.

Again, all depends on how the tree’s being put together… I’d likely purchase a dormant known cultivar (variety) on a known rootstock and immediately graft on a second cultivar to a couple of its ‘year old’ limbs – given it were at least a couple years old. Others are purists, insisting they build their own and graft it accordingly…

“2) If the answer to #1 is 'b', then is there any rhyme or reason as to which of the two varieties you should start with?”

2) I’d start with your (assumed) favorite variety as a base tree. Or, the better pollinator of the two, assuming whatever you ‘stuck on’ would likely get pollinated.

“3) I've never seen any reference to combo persimmon or quince. Is it possible to graft 2nd varieties onto either of these?”

3) Probably little interest, it’s difficult enough to get folks to try, let alone plant a persimmon or quince It’s possible, but (to me) less advised on the persimmon. Persimmons have a strange way of ‘shedding’ older limbs; they just dry up and fall off. Unless the second variety becames a major scaffold limb it’s productive life would be limited… It’s possible, but since pollination’s no problem for persimmons, there’s little need for a second cultivar – unless you want seeds…

The Quince would be easy, even fun, so why stop at two? …I don’t think they need pollinizers either… the old one I had didn’t, unless a Bartlett pear was doing the trick..?

“4) And lastly, is it possible to graft 2nd varieties onto established grape vines?”

4) I’ve never grafted grapes, only ‘played around’ with the cuts. Grapes are so easy to start, even cheap to purchase, I’d never chance losing a desired variety by accidentally breaking or lopping it off a mass of vines... It’s tough enough following one cane back to its origin without dealing with grafts! Just hack the big one back and plug a couple more into the ground, as far apart as space allows.

“Thanks in advance.” -- You’re welcome, good questions.

Yes quinces are self-fertile. Most people under 50 don't know what they are. Persimmons are only slightly less obscure.
JohnS
PDX OR

Mitch,

Generally, the most vigorous variety is used as the base tree (on your desired rootstock). In the Apple world, “Golden (or Yellow) Delicious” is often used as a multi-grafted base tree. 1) it’s nearly self-fertile, 2) it’s plenty vigorous.

With everything restricted by the rootstock, yours is a good question… and a major problem I’ve noticed with multi-variety fruit trees. The base tree, by necessity, is vigorous; to keep down ‘overgrowth’ from a ‘more vigorous’ second (or third & fourth…) variety -- and, to allow the ‘full flow’ of nutrients to the other varieties (cultivars). Most of these trees end up with spindly additional grafts (varieties) -- while the base tree dominates.

I’ve had less of a problem with ‘my own’ combinations -- because I prune and train for balance. I suspect you’d do the same. …but for those who don’t … their ‘extra varieties’ are often lost to the shade of an aggressive base variety tree.