Custom field multi-value selection and display

[Scargo]

Hy All!

I have a little problem.
I building a site with Joomla 2.5 and VM2. It's will be my online mobilephone museum/catalog.
I want to use custom fields. Field name: network. Value: 900 mhz, 1800 mhz, ETACS, etc. In the backend/product site/custom fields - I choose the network and for exaple the 900 mhz. And the question: how can i select more value here, for example 900 mhz and 1800 mhz for a dualband phone?

Thx. Scargo