Not knowing color of your stock - are they all splashes? ... but bringing in good black hard feather stock, both sexes, was a good move. I started with all splits several years ago - so while only 25% of split to split breedings don't carry the silkied gene - you want to either test breed to identify if questionable splits don't carry the gene and remove them from the program or breed them to full silkied so that the offspring will be at least for sure splits. The latter is much easier. So I'm suggesting you continue to breed your questionables to a full silkied.
What you don't want to do is breed questionable splits or even splits to hard feather birds. Just going to end up with a lot more questionable birds that don't carry the gene to sort thru. Way ahead of the game to have known entities.
If it were me, and I had the pens (4+) to dedicate - I'd start putting the 2 silkied girls in with the black cockerel.
In second pen silkied cockerels with black pullet and all but 1 or 2 of the questionable split pullets. Those 2 questionable pullets would go in a third pen with your known split roo. Discard all the questionable cockerels. Origional pen will continue to give you a few full silkieds of both genders. Silkied F and Silkied M will give you offspring containing different line outcross for sure splits but only the Silkied F pen will be fore sure outcrossed unless you can identify the black hens eggs. Replace your origional split roo with the black hen's split cockerel and your hard black roos with a son of his. Keep the split pullets produced in the same pens. The following year you should have full silkied to replace the daddy.. Can introduce a new hard feather roo. Split girls are fine with a silkied roo. Only put the hard feather new roo with full silkied girls.
Are you familiar with the concept of a spiral breeding program? 3 pens plus a bachelor pen to grow out replacement cockerel candidates. The Roos are rotated each year with better sons replacing fathers in rotation but the girls stay in the same pen and are just retired after 2 full years. The difference here is the pullet offspring move over a pen and the roos get replaced from thier own offspring in the same pen except for the hard feather roo in the first pen which is used to bring in fresh blood. Hard roo, silkied pullets and the split pullet offspring go to a full silkied roo and the pullet offspring from the second pen go to the third. The full silkied pullets produced from pen 3 go to the new hard feather or split son of the last hard feather new roo. That's what I'd do anyway. Your mileage may differ.