Lace shawl, double joins to make Crescent shape

I am knitting the Victoria lace shawl by Anne-Lise MIgaard. I am on Chart C which binds the border to the live shawl stitches and binds off as you go. The shawl is a crescent shape and to turn the corners smoothly requires double and triple joins across 7 stitches on each corner of the shawl. That is, 3 double joins, one triple join and another 3 double joins. I am puzzled as to how I complete a double (or triple) join using the same stitch. Pattern says I should join to the same live shawl stitch twice for a double join. I have interpreted this as knitting 2 stitches through back of looping

the first pass, which joins the border to the shawl. Coming back again on the alternate row to make the double join, I have just knit into the back of the last stitch on my needle (previously the 2 stitches through back of loop now just one stitch) is this correct, or am I missing something here. Your help was invaluable last time. I hope you can help me again

This may be what you’re doing but just so I understand, the pattern is joining 2sts from the border to one stitch from the shawl. The is repeated 3 times, then 3sts from the border to one stitch from the shawl followed by a repeat of the double join. You’re knitting these sts (2 + 1 or 3 + 1) together through the back loop. That sounds correct.
It’s done on the second row although I would have expected it on the first joining row.

I’m not sure my explanation was full enough. The last stitch of the border lace joins with the live stitch on the shawl. (Row2 and 4 of the chart) These are knit together through back of loop, joining the two pieces together, making a single stitch on my working needlessly . To work double joins, according to the symbol explanation, I need to “join to the same live shawl stitch twice - 4 rows of chart C attached to the same shawl stitch. To work triple joins, join to the same shawl stitch 3 times - 6 rows of chart C attached to the same shawl stitch.“ I’m having a mental block over how I join to the same live shawl stitch two or three times. Having already used it for a join it’s now on the needle with the border stitches. Doesn’t it now form part of the border and isn’t a live shawl stitch any more?