Hi there, my ds did a dual placement from year 3 at primary school. We started him full time at a special school, which we won two tribunals to get him into and keep him in. It meant that the LEA had to taxi him across county boundaries, hence the tribunals. By year 3 the special school felt the right move would be to consider a dual placement with our local primary school, at the same time my ds was beginning to feel that he wanted to be closer to home and with his siblings, who were starting there. Our LEA were not keen on dual placements at all, some don't work, but our special schools experience was that, if the communication was good then they could be very successful. We started him 1 day ms and 4 days special and slowly incremented ms until by year 6, he was doing 4 days ms and 1 day special.
My ds adjusted to this arrangement very well and both schools were fully behind it and I would say it was hugely successful and both schools felt that as well. The communication between the schools was good and I would say both schools took a pride in the arrangement, because they put the child at the centre and he had the 'best of both'. Both schools gained as well. The special school embraced training the ms staff and did termly outreach visits into the ms school as well as welcoming the ms TA's in to work with my ds.
The special school was able to pick the brains of the ms staff as well about some of the newer interventions for literacy/numeracy etc. It also helped the special school develope their ms outreach programme as they had a better understanding of the problems of integrating into main-stream.
The key to success as well as good communication, (the staff from both schools met regularly to talk), was to increment the mainstream input slowly.
That way you can see what the problems are and make sure there is no deterioration. My ds has just finished year 7 in our local ms secondary school full time and I have no doubt that he would not have adjusted so well if he had not been exposed to ms at primary level. The special school is continuing with the outreach and come into the secondary school once a week to give specialist input. This package is unheard of, but I think the LEA bought in to it because it was cheaper than taxi-ing him to the nearest ms resource. I am not confident that a ms secondary is right for him yet because like you I think my ds is in that black-hole in the middle, the special schools that are left take kids that are more severe, or more complex, but his special needs are complex enough to make ms a real challenge for him. Dipping a toe in ms at primary level did help us to go into ms with a better idea about what the challenges would be and how to overcome them.
All this goes to show that it is possible to come up with creative solutions that fit the child, if the will is there. Some kids won't cope/like having a foot in both camps, but it offered us a smooth transition into ms. As the level of physio dropped for example we had time to notice if there was a drop in mobility and could have pulled back from the ms transition. We had time to notice whether he could access the curriculum, before increasing ms input etc. Most ms staff still have scanty knowledge of disability, so they have time to get up to speed, before they have full responsibility for your child.
Socially it was hard for him to integrate into ms, but I think this was because of his pd and mld, rather than the fact he wasn't at ms full time or didn't start in year 1, ms. He found it a comfort to keep contact with the special school where his real mutually reciprocating friendships were and where they could empathise with each others difficulties more.
Go for what you think is right for your child, there is 'no one size fits all' and if you feel a dual placement would be right, go for it, you have nothing to lose. Make sure you do it on timescales that suit your child too. Our LEA wanted our ds full-time ms within a year and we said 'no way' that was too fast for him and we would take it at his speed, amazingly they backed off as they knew we would go to tribunal if we had to.
Good luck, hope this helps.