Softbox advice.

I'm planning on buying one of those 24" softboxes with a speedlite mount. (ebay)

I've found one that looks great, but I'd like it even more more if I could get a second flash in there.

http://www.ebay.ca …_trksid=p3984.m​1436.l2649

Does this exist?

Look at various bracket made by Mr. elv.

There's a commercial solution which fits those requirements in the Lastolite Ezybox II (the II naming is VERY important if you want to fit 2 flashes in there)

It's a modular softbox system designed to fit one or two speedlights and can also be used with strobes by simply changing the adapter ring (the optional third component). It's rather more expensive than your listed item though; nearly 3 times the price.

Ezybox II softbox:
http://www.amazon.co.u​k …00AFV33QC/ref=p​d_sim_ce_2
or, if you're in the USA cos you're yet another who gives no indication of location, try:-
http://www.bhphotovide​o.com …1_ezybox_ii_squ​are_m.html

Ezybox II mounting bracket (NEEDED):
http://www.amazon.co.u​k …2G8/ref=pd_bxgy​_ce_text_y
or, USA source:-
http://www.bhphotovide​o.com …t_bracket_for_e​zybox.html