Hi there and welcome!
AFAIK it's rather unusual to see a design in advance. Depends on the artist though, I guess it's worth asking if they'd do it for money.
But, isn't the whole point of checking out shops and artists' portfolios to find out who you'd really want the tattoo from? As in, trust them enough that it'll be great? Which also defies the point of asking someone else from the shop to do it because you don't want to wait. Just because they work in the same shop doesn't mean they are equally good or have similar styles...there are of course always things that most decent tattooist can do. But those don't usually need too much of advance design work anyway...
What's the hurry? The tattoo is for life, if you think you've found the right tattoo artist, just enjoy the wait. Look at it like waiting for xmas as a child ;-)
Good luck!