Webform and multiple memberships with same Member Organisation
Currently you cannot use Webform-CiviCRM to sign members up for two membership types of the same organization concurrently, and this is true for the current release (4.10) and is unchanged in the upcoming v4.11.

What you described in your testing is working as designed. If a user signs up for type MEM1 and pays the fee, then comes back a year later and signs up for MEM2, it will change their membership type to MEM2, charge them the price for MEM2, and extend their expiration by a year. The only thing in your testing that made it look weird is that you did both of those things on the same day. Just wait a year between signups and it will make sense :)

If you do want to use a webform to sign members up for 2 different membership types, simply set them up in CiviCRM as belonging to different member organizations, and then the webform will understand it as a separate signup and not a renewal.