I don't think you can have both expressed at the same time. To have Lavender, you have to have two copies of the gene and it will show up as lav. You could possibly have dun split to lav, but it would show up as dun, not lav. I could be wrong, and if I am, I hope someone corrects me.
No. Sounds like you have one unified colorgene... A lavender bird can carry one or two copies of the dun gene but it probably wouldn't show. There may be a theoretical effect of the dun on the lavender but it would fall in the range of normal lavender.
