multiple async-shell-command?

[Tom Roche]

I'm running

me@it:~$ lsb_release -a
No LSB modules are available.
Distributor ID: LinuxMint
Description:    Linux Mint Debian Edition
Release:        1
Codename:       debian
me@it:~$ uname -a
Linux it 3.0.0-1-amd64 #1 SMP Sun Jul 24 02:24:44 UTC 2011 x86_64 GNU/Linux
me@it:~$ emacs-snapshot --version
GNU Emacs 24.0.92.1

I was just dired-ing a folder of PDFs, where I did

> & evince

to display one. I then tried to display another in the same way, but got

> A command is running. Kill it? (yes or no)

How can I run multiple simultaneous async-shell-command?
Or is there another/better way to do what I want?

TIA, Tom Roche <Tom_Roche@pobox.com>