Yarn substitute

I found a baby hat patter that requires super bulky #6 9 st x 12 rows = 4 inches on 9mm needles
I have dk worsted #4 18 st x 22 rows = 4 inches on 6 mm needles can I hold it double for the same tension or find a different pattern?

According to this source 2 strands of dk is about the same as Aran weight/worsted and 2 strands of worsted is about the same as bulky. https://elizabethsmithknits.com/2020/07/31/double-stranding-101/ I’m not clear which you’re asking about.

What is the pattern? If you’re making a blanket you could just use more stitches perhaps.

The only way to really know is by making a swatch using your chosen yarn held double. There’s an often, if not always noticeable difference in the final fabric between using a single strand of yarn and two, to get a certain tension. My feeling is it might not be soft/flexible enough to be comfortable for a baby, but again, a swatch would give you the best idea!