I would send two or more files using ACTION_SEND on Android as an excerpt below:
Intent(android.content.Intent.ACTION_SEND); emailIntent.setType(\"p
I believe it's impossible using GMail.
If you want to programmaticly send email manually (skipping the GMail client), then you could attach as many items as you want, using android-javamailer:
http://www.jondev.net/articles/Sending_Emails_without_User_Intervention_%28no_Intents%29_in_Android