Unfortunately we do not have multiselection list items in V1. You could have a textbox where multiple customer names can be typed with comma delimited. In SP1, we have custom control support using which you can develop & deploy custom controls for wit form that'll make it easy for users to select multiple customers in a listbox. But when that control is associated with a text field, it is still comma delimited so querying on such fields may be problematic. Also, it requires development of such control and distribution to users.
You could use links as you suggested. Either hyperlinks if users can type names, or related links if you can create a "customer workitem" with customer name in each workitem - now users can search for that name and add links. We are planning to expand linking functionalities in next release.