Using multiple url parameters to create sticky-table entries (no replies)

I'm very, very new to HAproxy (started trying it out an hour ago) and am
looking at setting up a sticky-table based on multiple url_params.

I've got it working using a single parameter, but I'm wondering if I can
set it up to create table entries (keys) based on multiple url_param
entries when found. For example using a URL like this:

http://example.com/details?email=jdoe@example.com&newsletter_id=12345

I'd like it to create a key based on the strings in both the email and
newsletter_id parameters. Right now if I use the following it'll use just
email or newsletter_id if the former isn't found:

stick on url_param(email) table detailspage
stick on url_param(newsletter_id) table detailspage

What the sticky-table currently looks like:

$ echo "show table detailspage" | socat unix:/var/run/haproxy.sock -
# table: detailspage, type: string, size:5242880, used:3
0x7f90f0cc9a24: key=12345 use=0 exp=596822 server_id=1
0x7f90f0cc9754: key=jdoe@example.com use=0 exp=593655 server_id=1
0x7f90f0cc9964: key=harrycarry@example.com use=0 exp=595455 server_id=2

Thanks.