Combining multiple skus of inventory

[DaniFoFo]

Is there an app in which I can have 2 SKUs; so in case 1 SKU is out of stock, the other can fill in as a replacement? I have inventory that is identical, but is just packed differently. It's a huge hassle to go in and constantly switch SKUs if one or the other is sold out so I hope to find an app that can handle this. Thank you!