Going to play devil's advocate here, FL....
Whilst I appreciate that you thoroughly enjoyed yourself last weekend, may I ask whether you actually feel that either Sioned or Bertie have issues with you that you *NEED* to do join-up? And if you still want to proceed, it is definitely something I wouldn't recommend doing more than once with either of them so don't make anything permanent.
I'm not knocking MR or KM as I love their ideas and principles and apply many of them myself (although I'm not a dedicated follower of any one NH discipline, I pick out the bits that I think apply to my situations) and if I was forced to choose I guess I'd pick them as my favourites but I will say this. When I helped out at one of Kelly's Perfect Manners weekends a couple of years ago, the course members used the site's own horses to learn and practise join-up. Now, sorry if this sounds cynical but I think that those horses have learned the drill... they know that they will have to trot around the edge of the pen a few times and then when the human in the middle turns their back to them, they turn in and walk up to them and get a big pat. Similarly, people who take their own horses take them into the scary pen where loads of people are standing around the edge of it watching, send them away so to me, their horses are BOUND to turn in and approach their owners when the pressure is off... in that situation their human is the only thing they know... and anyone who has paid to go on the course AND go to the bother of taking their horse is sure to have a half-decent relationship with it before they've even started.
Don't get me wrong, I thoroughly enjoyed watching people blossom from not being very confident and assertive around horses to having their morales boosted and seeing them blossom into far more confident handlers. This is not a knock in any shape or form, just my observations.
Do have a go at join-up but I think you'll find that your two may get confused by the initial send-away when it isn't something you've done from the start.
And if you do try it, do let us know how you get on.
PS. When I loaned Buster, a 17.2 WB last year I would feed him and Roo in the field. Buzz would always finish his feed first and would then try snaking around to steal Roo's so I would send him away. Eventually Roo would finish and would wander away from his (very empty!) bucket but Buzz would always want to "make friends" with me again before he even thought about trying to see if Roo had left him any leftovers... it was quite a startling revelation and just shows that it can be done loose in the field.