What is your goal of this spawn?
Here is what I understand:
If you breed a petstore veiltail to a a doubletail, you will probably get veiltails and maybe some doubletail veiltails if the female carries doubletail, which is recesssive.
If you breed a crowntail petstore betta to a doubletail, combtails and a few doubletails may show up if both parents carry doubletail and crowntail, both of which are recessive.
Crowntail crosses can result in really messy finnage when mixed with veiltail. I understand if you have a special pet that you want to continue a special trait, or just let your pet live on through the natural process of life. I hope this helps. Cheers!