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[Auxin]

 
Well he responded. I was guessing his example was a ~6 month old werdermannianus or similar (on too short of a stock).
The above pic is a 3 year old Melocactus graft that hasnt grown in a year.
About 9 months after I would have started thinking the graft union was failing
That, combined with a response on shaman-australis, has left be being far less enthused
Back to the drawing board for my bowling ball Astrophytum at pereskiopsis speed.

Edit: Next wacky thought- wedge graft a 6+" Hylo to Peres, when growth is seen graft a Astro to the top. When growth stagnates cut off just above the Peres graft line and wedge graft the Hylo to a new Peres. Repeat until the hylo stock is too short to wedge graft and then root it.

[MeanGreen]

How about grafting your astro to like 4 or 5 (or more) pere stocks simultaneously?  If the scion is large enough to have its cambium cross the ones on all 4 stocks it is doable and has been done on SAB: http://www.shaman-australis.com/forum/index.php?/topic/44882-multiple-stock-grafting-using-pereskiopsis-an-experiment/

I'm thinking 5 large pereskiopsis with ample root systems should be able to give you your bowling ball or close to it.
You could even cut the side of each stock vertically, have them graft together and use that as a stock. Sort of like ahat some people do with bonsais, using several trees tied together until they fuse together.

[Auxin]

I've thought about that, and even tried it once, but theyd have to plug into an intermediate permanent stock anyway.
When the graft fails I've had it cause rot to go up the inside of the scion and its not good practice, in my opinion, to leave a stump of pereskiopsis in a degrafted globular. A permanent stock base (jusbertii, hylo, myrtilo, or of course Opuntia!) could graft to multiple peres, but successive single grafts would be easier from a practical standpoint.
Still, it would be funny to graft an opuntia to peres on bottom and globular on top then when it fails graft in a new peres and wait until it succeeds before degrafting the old and just have the round pad kind of go back and forth being grafted on one side or the other, mounted to a post in the middle. A time lapse video would be hilarious.

[Inyan]

Auxin: After seeing the graft in question... I have a few that look like that. Not sure I would accept that as anything worth trying to achieve. Especially so at that height.

MeanGreen: Multiple grafts are fine, but do they really push any more into the scion? Each stock also provides for an additional matter of pruning. When all is said and done, I'd be willing to bet dollars to donuts that at a certain size the stock is going to max out growth on any scion and no more will be gained from any additional sourced stock plugged into the side.

And yes, I've experimented with multiple stocks for scions with both Brugmansia and cacti in general. From a practical standpoint, if you want something that looks creative and you want to have multiple different colored or formed flowers it may make sense... but then you are also pumping out multiple scions up top as well. Any one of which could fail as often times the stock seems to pick one scion over another to push hard and the others may grow slow or even stop growing altogether after a time.

[CactusGrower]

Not quiabentia, they do look close but I'm 99% sure this is Pereskiopsis aquosa a.k.a Jumbo Pere, either way I'm propagating the hell out of it. Also have some variegated P. diguetti.

Edit: @Inyan I was just kidding, and that's a very impressive stock you have there I would love to see how much growth a pere that big would pump in a seedling.

Hit me up if you want a few cuts of the jumbo, guys.

Are you  still propagating a lot of the Pereskiopsis aquosa ? Its really hard to find, especially in Canada