In general - when I have mixed different weights of yarn - I've adjusted the sett for each yarn I'm using as I go across designing, threading and denting. For instance - 40/2 linen is often sett at 30 epi and 16/2 at 15 epi. If you don't factor the different setts into your design, your width on the loom might be off. When designing - I might try using the 40/2 as a background and the other linen as the huck - I'm thinking border or frame on the mat. Since this is an important piece for you and your family - I probably would do a sample first, to see if it's going to give me the look I have in mind. Keep us posted and we'd love to see a photo when you've finished it.