I do this and do ssh-add as part of my .bash_profile:
sudo -u otherusername ssh-agent bash -l
Unfortunately, I have no history when I use the up arrow in doing this. However, I execute the same command minus the sudo part.
By the way, this is what I have in my bash profile to automatically run ssh-agent (only once) and add the RSA key (only once):
# Start ssh-agent & add key if [[ -z $SSH_AGENT_PID ]]; then echo Starting ssh-agent automatically... ssh-agent bash -l elif [[ 0 == `ssh-add -l | grep "(RSA)" -c` ]]; then echo Adding ssh key automatically...
source share