Assembla FAQ and Documentation

You need unique key per each user anyway
There is little workaround to be able change keys for assembla.com for different project.

1)place somewhere in $PATH this script (let its name will be gitssh)

#!/bin/sh
key=$(git config ssh.key)

if [ -z "$key" ]; then
  git_opts=""
else
  git_opts="-i ${key}"
fi

exec ssh $git_opts "$@"

2)
set env variable GIT_SSH point to this script

export GIT_SSH=gitssh

3) configure git per project per key
in project1 directory

git config ssh.key ~/.ssh/my_cool_project1_key.pub

in project2 directory

git config ssh.key ~/.ssh/my_cool_project2_key.pub