Hi Nico. Grow your F1 and save lots of seeds. You only need one F1 plant since they are all genetically the same with half and half genes from each parent.
As Ginger said, with multiflora being a simple recessive trait, the expected ratio is one in four plants with that trait. Actual probability of finding one runs a little differently, and better to grow a half dozen plants to be fairly certain to get one multiflora. As long as there is no linkage involved, you can be pretty sure that six is enough to get one.
I guess you can select early for multiflora, as soon as the first cluster is formed. Even starting a lot of plants in small containers, if you can get them to that stage before planting out, you can keep only multifloras, and won't waste any space.
