Yamaha DTXpress/DTXplorer/DTXtreme

Hi all, recently bought myself a dtxpress 2 kit (used to play drums in 
a college band 20 years ago, thought I'd get back into drumming again) 
and searching on the net found this site - an excellent source of info!

Wanting to expand  my kit and wondered could I attach a Yamaha DD55 via 
midi out on the DD55 to midi in on the DTX 2  module ( or is it the 
other way round?), - would this allow me to assign sounds from the DTX 
2 module to the 7 pads on the DD55 and hear them when i hit the pads on 
the DD55 - or have I got the whole midi thing wrong?

any help appreciated, thanks