extending a roaming network

This will work with one Express, and I can think of no reason it should not work with two as you proposed. I have not tried it with two. I recommend you deploy one first, verify that it works, then add the second. Since you already have a roaming network the hard part is over.

This KB article explains the details: http://support.apple.com/kb/HT4259

Note the recommendation:

If you have configured your Wi-Fi base stations in the past, it may be helpful to do a factory default reset of each Wi-Fi base station that will be part of the wireless extended network before you begin.

... though I do not believe this will be necessary.

Will I need to change any of the settings to the base stations to do this in a roaming network and how should I configure each express unit?

Configure each AEBS to "allow this network to be extended"

... and the Expresses to "extend a wireless network" and allow wireless clients:

That should be all there is to it.

Pease let me know how this works.