How to get my dog to stop barking when I go upstairs to bed?

I have a 4 month old chihuahua dachsund mix that is not allowed upstairs. we know he has a separation anxiety problem that is well getting worked on but for now we are using a gate to keep them in the kitchen at night- don't worry, the kitchen is a pretty big room for them to sniff around, and we only keep them in there at night. One night I was struggling to get them to sleep so I put my robe on the rug for Oscar (my dog) and it seemed to really calm him down then I sneeked out of the kitchen without him seeing me and they also have a moist towel in their crate which also calmed him down. When I went upstairs I didn't hear any barking or crying. That night was okay so I pretty much solved my problem. Just in case my plan does not last what are some other Ideas? HINT: I cannot use a kong because I have 2 dogs down there and only one kong and I cannot lock him in his crate.