Very Disruptive Dog Barking In Elevator?

Hello,
I have two dogs, a Havanese and a new Mini Dachshund (now 6 months old). Normally they are great, but my Mini Dachshund barks extrememly frantically and loudly whenever another dog enters an elevator with us and sometimes when another dog passes closely by in a building or outside. I have trained him to almost entirely stop barking at dogs outside by stopping him from walking whenever he starts to get too excited.
My problem is that I can’t seem to stop him from barking in the elevator (I have even tried to hold him mouth shut – but I don’t want to hurt him accidentally) and I am having a very hard time socializing him because if I let him get close enough to another dog (other than my own Havanese) he will attempt to nip.
Any suggestion are appreciated. I know Dachshunds are known to be barkers, but his reaction in the elevator to other dogs seems extreme.

What Dog Should I Get Between A Maltese Or A Havanese, They Are Both Very Similar?

I like them both very much and find it hard to decide
I prefer the Havanese as they come in different colours
But i also like the Maltese as they are smaller
I want a playful, lively, affectionate dog which likes indoors